diff -u b/src/Entity/MediaBundle.php b/src/Entity/MediaBundle.php
--- b/src/Entity/MediaBundle.php
+++ b/src/Entity/MediaBundle.php
@@ -7,8 +7,10 @@
 use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\Core\Entity\EntityWithPluginCollectionInterface;
 use Drupal\Core\Plugin\DefaultSingleLazyPluginCollection;
+use Drupal\field\FieldStorageConfigInterface;
 use Drupal\media_entity\MediaBundleInterface;
 use Drupal\media_entity\MediaInterface;
+use Drupal\media_entity\SourceFieldInterface;
 
 /**
  * Defines the Media bundle configuration entity.
@@ -249,8 +251,50 @@
     parent::postSave($storage, $update);
 
     if (!$update) {
-      // Inform the plugin that this bundle has just been created.
-      $this->getType()->onBundleSave($this->id());
+      $type_plugin = $this->getType();
+
+      if ($type_plugin instanceof SourceFieldInterface) {
+        $field = $type_plugin->getSourceField($this);
+
+        $storage = $field->getFieldStorageDefinition();
+        if ($storage instanceof FieldStorageConfigInterface) {
+          $storage->save();
+        }
+
+        if ($field->isNew()) {
+          $entity_type = $field->getTargetEntityTypeId();
+          $bundle = $field->getTargetBundle();
+
+          $plugin_definition = \Drupal::service('plugin.manager.field.field_type')
+            ->getDefinition($field->getType());
+
+          if ($field->isDisplayConfigurable('form')) {
+            $widget = \Drupal::service('plugin.manager.field.widget')
+              ->createInstance($plugin_definition['default_widget'])
+              ->getConfiguration();
+
+            entity_get_form_display($entity_type, $bundle, 'default')
+              ->setComponent($field->getName(), [
+                'type' => $widget->getPluginId(),
+                'settings' => $widget->getConfiguration(),
+              ])
+              ->save();
+          }
+          if ($field->isDisplayConfigurable('view')) {
+            $formatter = \Drupal::service('plugin.manager.field.formatter')
+              ->createInstance($plugin_definition['default_formatter']);
+
+            entity_get_display($entity_type, $bundle, 'default')
+              ->setComponent($field->getName(), [
+                'type' => $formatter->getPluginId(),
+                'settings' => $formatter->getConfiguration(),
+              ])
+              ->save();
+          }
+        }
+
+        $field->save();
+      }
     }
 
   }
diff -u b/src/MediaTypeBase.php b/src/MediaTypeBase.php
--- b/src/MediaTypeBase.php
+++ b/src/MediaTypeBase.php
@@ -7,6 +7,7 @@
 use Drupal\Core\Config\Config;
 use Drupal\Core\Entity\EntityFieldManagerInterface;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Field\FieldDefinitionInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
@@ -15,7 +16,7 @@
 /**
  * Base implementation of media type plugin.
  */
-abstract class MediaTypeBase extends PluginBase implements MediaTypeInterface, ContainerFactoryPluginInterface {
+abstract class MediaTypeBase extends PluginBase implements MediaTypeInterface, SourceFieldInterface, ContainerFactoryPluginInterface {
   use StringTranslationTrait;
 
   /**
@@ -105,7 +106,9 @@
    * {@inheritdoc}
    */
   public function defaultConfiguration() {
-    return [];
+    return [
+      'source_field' => NULL,
+    ];
   }
 
   /**
@@ -138,7 +141,31 @@
    * {@inheritdoc}
    */
   public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
-    return [];
+    $options = [];
+
+    /** @var \Drupal\media_entity\MediaBundleInterface $bundle */
+    $bundle = $form_state->getFormObject()->getEntity();
+    foreach ($this->entityFieldManager->getFieldDefinitions('media', $bundle->id()) as $field_name => $field) {
+      $allowed_type = in_array($field->getType(), $this->pluginDefinition['allowed_field_types']);
+      $is_base_field = $field->getFieldStorageDefinition()->isBaseField();
+      if ($allowed_type && !$is_base_field) {
+        $options[$field_name] = $field->getLabel();
+      }
+    }
+
+    // Select the source field. Only show it when the bundle is not new, so
+    // there will potentially be fields to select.
+    $form['source_field'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Field with source information.'),
+      '#default_value' => $this->configuration['source_field'],
+      '#empty_option' => $this->t('- create -'),
+      '#empty_value' => NULL,
+      '#options' => $options,
+      '#disabled' => empty($options),
+    ];
+
+    return $form;
   }
 
   /**
@@ -162,5 +189,50 @@
    * {@inheritdoc}
    */
-  public function onBundleSave($bundle_name) {}
+  public function getSourceField(MediaBundleInterface $bundle) {
+    $id = 'media.' . $bundle->id() . '.' . $this->configuration['source_field'];
+
+    return $this->entityTypeManager
+      ->getStorage('field_config')
+      ->load($id)
+      ?:
+      $this->createSourceField($bundle);
+  }
+
+  /**
+   * Creates the source field definition for a bundle.
+   *
+   * @param \Drupal\media_entity\MediaBundleInterface $bundle
+   *   The bundle.
+   *
+   * @return \Drupal\field\FieldConfigInterface
+   *   The unsaved field definition. The field storage definition, if new,
+   *   should not be saved either.
+   */
+  abstract protected function createSourceField(MediaBundleInterface $bundle);
+
+  /**
+   * Determine a free field name to use as the default field.
+   *
+   * @return string
+   *   An appropriate field name that was determined to be available.
+   */
+  protected function getSourceFieldName() {
+    $base_id = 'media.field_media_' . $this->getPluginId();
+    $tries = 0;
+    $storage = $this->entityTypeManager->getStorage('field_storage_config');
+
+    // Iterate at least once, until no field with the generated ID is found.
+    do {
+      $id = $base_id;
+      // If we've tried before, increment and append  suffix.
+      if ($tries) {
+        $id .= '_' . ++$tries;
+      }
+      $field = $storage->load($id);
+    }
+    while ($field);
+
+    return $id;
+  }
 
 }
diff -u b/src/MediaTypeInterface.php b/src/MediaTypeInterface.php
--- b/src/MediaTypeInterface.php
+++ b/src/MediaTypeInterface.php
@@ -87,10 +87,2 @@
 
-  /**
-   * Take actions when saving a bundle.
-   *
-   * @param string $bundle_name
-   *   The machine-name of the bundle saved.
-   */
-  public function onBundleSave($bundle_name);
-
 }
reverted:
--- b/src/MediaTypeSourceFieldTrait.php
+++ /dev/null
@@ -1,233 +0,0 @@
-<?php
-
-namespace Drupal\media_entity;
-
-use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\Entity\Entity\EntityFormDisplay;
-use Drupal\Core\Entity\Entity\EntityViewDisplay;
-use Drupal\field\Entity\FieldConfig;
-use Drupal\field\Entity\FieldStorageConfig;
-
-/**
- * To exhibit by derivatives of Drupal\media_entity\MediaTypeBase.
- *
- * Implements most of the behavior required for a single source field that may
- * be populated with a default field upon creation of a bundle.
- *
- * You will need to add a source_field key to your configuration schema and
- * annotate your class with the MediaTypeSourceField annotation.
- */
-trait MediaTypeSourceFieldTrait {
-
-  /**
-   * The default source field name.
-   *
-   * @var string
-   */
-  protected $defaultSourceFieldName;
-
-  /**
-   * Ensures the t() method is available.
-   *
-   * @see \Drupal\Core\StringTranslation\StringTranslationTrait
-   */
-  abstract protected function t($string, array $args = [], array $options = []);
-
-  /**
-   * Provide information related to source field configuration.
-   *
-   * This includes information about a default field, a field that will be
-   * created automatically if the user does not disable the automatich creation.
-   *
-   * @return array[array]
-   *   An array containing keys identifying information regarding the source
-   *   field configuration:
-   *   - allowed_types: array of strings specifying field types that are
-   *     acceptable as source field types.
-   *   - default_type: string identifying the default field type.
-   *   - default_label: string that will be used for the label for the default
-   *     field.
-   *   - default_widget: string identifying the form widget for the default
-   *     field.
-   *   - default_formatter: string identifying the formatter to be used in the
-   *     display.
-   *   - field_storage_extra_config: (optional) array with extra keys to be
-   *     passed to FieldStorageConfig::create().
-   *   - field_extra_config: (optional) array with extra keys to be passed to
-   *     FieldConfig::create().
-   */
-  abstract protected function sourceFieldInfo();
-
-  /**
-   * Produce a form fragment to configure the source field.
-   *
-   * To be called from a media type's buildConfigurationForm() method.
-   *
-   * @param array $form
-   *   The $form array as passed to the buildConfigurationForm().
-   * @param \Drupal\Core\Form\FormStateInterface $form_state
-   *   The $form_state instance.
-   *
-   * @return array
-   *   The form element to be used in the configuration form.
-   */
-  protected function sourceFieldConfigurationForm(array $form, FormStateInterface $form_state) {
-    $source_field_info = $this->sourceFieldInfo();
-    $options = [];
-
-    $allowed_field_types = $source_field_info['allowed_types'];
-    /** @var \Drupal\media_entity\MediaBundleInterface $bundle */
-    $bundle = $form_state->getFormObject()->getEntity();
-    foreach ($this->entityFieldManager->getFieldDefinitions('media', $bundle->id()) as $field_name => $field) {
-      $allowed_type = in_array($field->getType(), $allowed_field_types);
-      $is_base_field = $field->getFieldStorageDefinition()->isBaseField();
-      if ($allowed_type && !$is_base_field) {
-        $options[$field_name] = $field->getLabel();
-      }
-    }
-
-    // Add a checkbox to allow the field being created automatically on save.
-    // Only include it when the bundle is new, so we can use its presence to
-    // know we can safely override the source field configuration.
-    if ($bundle->isNew()) {
-      $form['create_source_field'] = [
-        '#type' => 'checkbox',
-        '#title' => $this->t('Create a source field automatically when saving this form.'),
-        '#description' => $this->t('If checked, a default field will be created and used as a source field. If you uncheck the field, you will need to create a field and revisit this form later to select it.'),
-        '#default_value' => TRUE,
-      ];
-    }
-
-    // Select the source field. Only show it when the bundle is not new, so
-    // there will potentially be fields to select.
-    $form['source_field'] = [
-      '#type' => 'select',
-      '#title' => $this->t('Field with source information'),
-      '#default_value' => isset($this->configuration['source_field']) ? $this->configuration['source_field'] : '',
-      '#empty_option' => $this->t('- None -'),
-      '#empty_value' => '',
-      '#options' => $options,
-      '#access' => !$bundle->isNew(),
-    ];
-
-    return $form;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function onBundleSave($bundle_name) {
-    // If we are creating a new bundle and the user elected to have the default
-    // field created, defaultSourceFieldName will have been set from
-    // setConfiguration() and determineDefaultSourceFieldName().
-    // @see determineDefaultSourceFieldName()
-    if (isset($this->defaultSourceFieldName)) {
-      $this->createDefaultSourceField($bundle_name);
-    }
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function createDefaultSourceField($bundle_name) {
-    $source_field_info = $this->sourceFieldInfo();
-
-    if (!in_array($source_field_info['default_type'], $source_field_info['allowed_types'])) {
-      throw new \LogicException('Default source field type is not within the allowed field types.');
-    }
-
-    $field_name = $this->defaultSourceFieldName;
-
-    $storage = FieldStorageConfig::create([
-      'field_name' => $field_name,
-      'entity_type' => 'media',
-      'type' => $source_field_info['default_type'],
-    ]);
-    $storage->save();
-
-    // Create the field instance.
-    FieldConfig::create([
-      'entity_type' => 'media',
-      'field_name' => $field_name,
-      'label' => $source_field_info['default_label'],
-      'required' => TRUE,
-      'bundle' => $bundle_name,
-    ])->save();
-
-    // Make the field visible on the form display.
-    /** @var \Drupal\Core\Entity\Display\EntityFormDisplayInterface $form_display */
-    $form_display = EntityFormDisplay::create([
-      'targetEntityType' => 'media',
-      'bundle' => $bundle_name,
-      'mode' => 'default',
-      'status' => TRUE,
-    ]);
-    $form_display->setComponent($field_name, [
-      'type' => $source_field_info['default_widget'],
-    ])->save();
-
-    // Make the field visible on the media entity itself.
-    /** @var \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display */
-    $display = EntityViewDisplay::create([
-      'targetEntityType' => 'media',
-      'bundle' => $bundle_name,
-      'mode' => 'default',
-      'status' => TRUE,
-    ]);
-    $display->setComponent($field_name, [
-      'type' => $source_field_info['default_formatter'],
-    ])->save();
-  }
-
-  /**
-   * Determine a free field name to use as the default field.
-   *
-   * @return string
-   *   An appropriate field name that was determined to be available.
-   */
-  protected function determineDefaultSourceFieldName() {
-    // Determine a unique field name.
-    if (isset($this->defaultSourceFieldName)) {
-      return $this->defaultSourceFieldName;
-    }
-
-    $base_field_name = 'field_media_' . $this->getPluginId();
-    $field_name_candidate = $base_field_name;
-    $postfix = '';
-    while (FieldStorageConfig::loadByName('media', $field_name_candidate)) {
-      if (empty($postfix)) {
-        $postfix = 1;
-      }
-      else {
-        $postfix++;
-      }
-
-      $field_name_candidate = $base_field_name . '_' . $postfix;
-    }
-
-    $this->defaultSourceFieldName = $field_name_candidate;
-
-    return $this->defaultSourceFieldName;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setConfiguration(array $configuration) {
-    // Take out the create_source_field key when present so it doesn't end up
-    // in the configuration. Record it was set so we can later decide to
-    // actually create the field. Determine the field name here so it correctly
-    // is entered into the configuration.
-    if (isset($configuration['create_source_field'])) {
-      $create_source_field = $configuration['create_source_field'];
-      unset($configuration['create_source_field']);
-
-      if ($create_source_field) {
-        $configuration['source_field'] = $this->determineDefaultSourceFieldName();
-      }
-    }
-
-    parent::setConfiguration($configuration);
-  }
-
-}
only in patch2:
unchanged:
--- a/src/Annotation/MediaType.php
+++ b/src/Annotation/MediaType.php
@@ -40,4 +40,11 @@ class MediaType extends Plugin {
    */
   public $description = '';
 
+  /**
+   * The field types that can be used as a source field for this type.
+   *
+   * @var string[]
+   */
+  public $allowed_field_types = [];
+
 }
only in patch2:
unchanged:
--- a/src/Plugin/MediaEntity/Type/Generic.php
+++ b/src/Plugin/MediaEntity/Type/Generic.php
@@ -3,6 +3,7 @@
 namespace Drupal\media_entity\Plugin\MediaEntity\Type;
 
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\media_entity\MediaBundleInterface;
 use Drupal\media_entity\MediaInterface;
 use Drupal\media_entity\MediaTypeBase;
 
@@ -50,4 +51,27 @@ class Generic extends MediaTypeBase {
     return $form;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function createSourceField(MediaBundleInterface $bundle) {
+    $field_storage = $this->entityTypeManager
+      ->getStorage('field_storage_config')
+      ->create([
+        'entity_type' => 'media',
+        'field_name' => $this->getSourceFieldName(),
+        // Strings are harmless, inoffensive puppies: a good choice for a
+        // generic media type.
+        'type' => 'string',
+      ]);
+
+    /** @var \Drupal\field\FieldConfigInterface $field */
+    return $this->entityTypeManager
+      ->getStorage('field_config')
+      ->create([
+        'field_storage' => $field_storage,
+        'bundle' => $bundle->id(),
+      ]);
+  }
+
 }
only in patch2:
unchanged:
--- /dev/null
+++ b/src/SourceFieldInterface.php
@@ -0,0 +1,20 @@
+<?php
+
+namespace Drupal\media_entity;
+
+/**
+ * Interface for media type plugins that depend on a field.
+ */
+interface SourceFieldInterface {
+
+  /**
+   * Returns the source field for a bundle using this plugin.
+   *
+   * @param \Drupal\media_entity\MediaBundleInterface $bundle
+   *   The media bundle.
+   *
+   * @return \Drupal\field\FieldConfigInterface
+   */
+  public function getSourceField(MediaBundleInterface $bundle);
+
+}