diff --git a/core/core.services.yml b/core/core.services.yml
index d2aa505..39b0a04 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -569,8 +569,8 @@ services:
   entity.autocomplete_matcher:
     class: Drupal\Core\Entity\EntityAutocompleteMatcher
     arguments: ['@plugin.manager.entity_reference_selection']
-  block_form.manager:
-    class: Drupal\Core\Block\BlockFormManager
+  plugin_form.manager:
+    class: Drupal\Core\Plugin\PluginFormManager
     arguments: ['@class_resolver']
   plugin.manager.entity_reference_selection:
     class: Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginManager
diff --git a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
index 06365fa..8e9e77a 100644
--- a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
+++ b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
@@ -239,9 +239,20 @@ public function useCaches($use_caches = FALSE) {
    * method.
    */
   public function processDefinition(&$definition, $plugin_id) {
+    // Only arrays can be operated on.
+    if (!is_array($definition)) {
+      return;
+    }
+
     if (!empty($this->defaults) && is_array($this->defaults)) {
       $definition = NestedArray::mergeDeep($this->defaults, $definition);
     }
+
+    // If no default form is defined and this plugin implements
+    // \Drupal\Core\Plugin\PluginFormInterface, use that for the default form.
+    if (!isset($definition['form']['default']) && isset($definition['class']) && is_subclass_of($definition['class'], PluginFormInterface::class)) {
+      $definition['form']['default'] = $definition['class'];
+    }
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Block/BlockFormManager.php b/core/lib/Drupal/Core/Plugin/PluginFormManager.php
similarity index 84%
rename from core/lib/Drupal/Core/Block/BlockFormManager.php
rename to core/lib/Drupal/Core/Plugin/PluginFormManager.php
index f866706..dec70ba 100644
--- a/core/lib/Drupal/Core/Block/BlockFormManager.php
+++ b/core/lib/Drupal/Core/Plugin/PluginFormManager.php
@@ -1,17 +1,16 @@
 <?php
 
-namespace Drupal\Core\Block;
+namespace Drupal\Core\Plugin;
 
 use Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException;
 use Drupal\Component\Plugin\PluginInspectionInterface;
 use Drupal\Core\DependencyInjection\ClassResolverInterface;
-use Drupal\Core\Plugin\PluginFormInterface;
 use Drupal\Core\Form\OperationAwareFormInterface;
 
 /**
- * Provides form discovery capabilities for block plugins.
+ * Provides form discovery capabilities for plugins.
  */
-class BlockFormManager implements BlockFormManagerInterface {
+class PluginFormManager implements PluginFormManagerInterface {
 
   /**
    * The class resolver.
@@ -21,7 +20,7 @@ class BlockFormManager implements BlockFormManagerInterface {
   protected $classResolver;
 
   /**
-   * BlockFormManager constructor.
+   * PluginFormManager constructor.
    *
    * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
    */
@@ -32,13 +31,13 @@ public function __construct(ClassResolverInterface $class_resolver) {
   /**
    * {@inheritdoc}
    */
-  public function getFormObject(PluginInspectionInterface $plugin, $operation) {
+  public function getFormObject(PluginInspectionInterface $plugin, $operation, $fallback_operation = NULL) {
     $definition = $plugin->getPluginDefinition();
 
     if (!isset($definition['form'][$operation])) {
       // Use the default form class if no form is specified for this operation.
-      if (isset($definition['form']['default'])) {
-        $operation = 'default';
+      if ($fallback_operation && isset($definition['form'][$fallback_operation])) {
+        $operation = $fallback_operation;
       }
       else {
         throw new InvalidPluginDefinitionException($plugin->getPluginId(), sprintf('The "%s" plugin did not specify a "%s" form class', $plugin->getPluginId(), $operation));
diff --git a/core/lib/Drupal/Core/Block/BlockFormManagerInterface.php b/core/lib/Drupal/Core/Plugin/PluginFormManagerInterface.php
similarity index 59%
rename from core/lib/Drupal/Core/Block/BlockFormManagerInterface.php
rename to core/lib/Drupal/Core/Plugin/PluginFormManagerInterface.php
index 2135128..6038d61 100644
--- a/core/lib/Drupal/Core/Block/BlockFormManagerInterface.php
+++ b/core/lib/Drupal/Core/Plugin/PluginFormManagerInterface.php
@@ -1,13 +1,13 @@
 <?php
 
-namespace Drupal\Core\Block;
+namespace Drupal\Core\Plugin;
 
 use Drupal\Component\Plugin\PluginInspectionInterface;
 
 /**
- * Provides form discovery capabilities for block plugins.
+ * Provides an interface for retrieving form objects for plugins.
  */
-interface BlockFormManagerInterface {
+interface PluginFormManagerInterface {
 
   /**
    * Creates a new form instance.
@@ -15,13 +15,15 @@
    * @param \Drupal\Component\Plugin\PluginInspectionInterface $plugin
    *   The plugin the form is for.
    * @param string $operation
-   *   The name of the operation to use, e.g., 'default'.
+   *   The name of the operation to use, e.g., 'add' or 'edit'.
+   * @param string $fallback_operation
+   *   (optional) The name of the fallback operation to use.
    *
    * @return \Drupal\Core\Plugin\PluginFormInterface
    *   A plugin form instance.
    *
    * @throws \Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException
    */
-  public function getFormObject(PluginInspectionInterface $plugin, $operation);
+  public function getFormObject(PluginInspectionInterface $plugin, $operation, $fallback_operation = NULL);
 
 }
diff --git a/core/modules/block/src/BlockForm.php b/core/modules/block/src/BlockForm.php
index 38272bd..c96b4e8 100644
--- a/core/modules/block/src/BlockForm.php
+++ b/core/modules/block/src/BlockForm.php
@@ -3,7 +3,7 @@
 namespace Drupal\block;
 
 use Drupal\Component\Utility\Html;
-use Drupal\Core\Block\BlockFormManagerInterface;
+use Drupal\Core\Plugin\PluginFormManagerInterface;
 use Drupal\Core\Block\BlockPluginInterface;
 use Drupal\Core\Entity\EntityForm;
 use Drupal\Core\Entity\EntityManagerInterface;
@@ -71,11 +71,11 @@ class BlockForm extends EntityForm {
   protected $contextRepository;
 
   /**
-   * The block form manager.
+   * The plugin form manager.
    *
-   * @var \Drupal\Core\Block\BlockFormManagerInterface
+   * @var \Drupal\Core\Plugin\PluginFormManagerInterface
    */
-  protected $blockFormManager;
+  protected $pluginFormManager;
 
   /**
    * Constructs a BlockForm object.
@@ -90,16 +90,16 @@ class BlockForm extends EntityForm {
    *   The language manager.
    * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
    *   The theme handler.
-   * @param \Drupal\Core\Block\BlockFormManagerInterface $block_form_manager
-   *   The block form manager.
+   * @param \Drupal\Core\Plugin\PluginFormManagerInterface $plugin_form_manager
+   *   The plugin form manager.
    */
-  public function __construct(EntityManagerInterface $entity_manager, ExecutableManagerInterface $manager, ContextRepositoryInterface $context_repository, LanguageManagerInterface $language, ThemeHandlerInterface $theme_handler, BlockFormManagerInterface $block_form_manager) {
+  public function __construct(EntityManagerInterface $entity_manager, ExecutableManagerInterface $manager, ContextRepositoryInterface $context_repository, LanguageManagerInterface $language, ThemeHandlerInterface $theme_handler, \Drupal\Core\Plugin\PluginFormManagerInterface $plugin_form_manager) {
     $this->storage = $entity_manager->getStorage('block');
     $this->manager = $manager;
     $this->contextRepository = $context_repository;
     $this->language = $language;
     $this->themeHandler = $theme_handler;
-    $this->blockFormManager = $block_form_manager;
+    $this->pluginFormManager = $plugin_form_manager;
   }
 
   /**
@@ -112,7 +112,7 @@ public static function create(ContainerInterface $container) {
       $container->get('context.repository'),
       $container->get('language_manager'),
       $container->get('theme_handler'),
-      $container->get('block_form.manager')
+      $container->get('plugin_form.manager')
     );
   }
 
@@ -435,7 +435,7 @@ public function getUniqueMachineName(BlockInterface $block) {
    *   The plugin form for the block.
    */
   protected function getPluginForm(BlockPluginInterface $block) {
-    return $this->blockFormManager->getFormObject($block, $this->operation);
+    return $this->pluginFormManager->getFormObject($block, 'default');
   }
 
 }
diff --git a/core/modules/block/tests/modules/block_test/src/Form/SecondaryBlockForm.php b/core/modules/block/tests/modules/block_test/src/Form/SecondaryBlockForm.php
index 3095963..c5aac97 100644
--- a/core/modules/block/tests/modules/block_test/src/Form/SecondaryBlockForm.php
+++ b/core/modules/block/tests/modules/block_test/src/Form/SecondaryBlockForm.php
@@ -7,7 +7,7 @@
 use Drupal\Core\Plugin\PluginFormInterface;
 
 /**
- * @todo.
+ * Provides a form that is used as a secondary form for a block.
  */
 class SecondaryBlockForm implements PluginFormInterface, OperationAwareFormInterface {
 
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestMultipleFormsBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestMultipleFormsBlock.php
index ff5ced3..01a8fe2 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestMultipleFormsBlock.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestMultipleFormsBlock.php
@@ -5,7 +5,7 @@
 use Drupal\Core\Block\BlockBase;
 
 /**
- * @todo.
+ * Provides a block with multiple forms.
  *
  * @Block(
  *   id = "test_multiple_forms_block",
diff --git a/core/modules/block/tests/src/Unit/BlockFormTest.php b/core/modules/block/tests/src/Unit/BlockFormTest.php
index a27014a..07b3780 100644
--- a/core/modules/block/tests/src/Unit/BlockFormTest.php
+++ b/core/modules/block/tests/src/Unit/BlockFormTest.php
@@ -3,7 +3,7 @@
 namespace Drupal\Tests\block\Unit;
 
 use Drupal\block\BlockForm;
-use Drupal\Core\Block\BlockFormManagerInterface;
+use Drupal\Core\Plugin\PluginFormManagerInterface;
 use Drupal\Tests\UnitTestCase;
 
 /**
@@ -56,11 +56,11 @@ class BlockFormTest extends UnitTestCase {
   protected $contextRepository;
 
   /**
-   * The block form manager.
+   * The plugin form manager.
    *
-   * @var \Drupal\Core\Block\BlockFormManagerInterface|\Prophecy\Prophecy\ProphecyInterface
+   * @var \Drupal\Core\Plugin\PluginFormManagerInterface|\Prophecy\Prophecy\ProphecyInterface
    */
-  protected $blockFormManager;
+  protected $pluginFormManager;
 
   /**
    * {@inheritdoc}
@@ -79,7 +79,7 @@ protected function setUp() {
       ->method('getStorage')
       ->will($this->returnValue($this->storage));
 
-    $this->blockFormManager = $this->prophesize(BlockFormManagerInterface::class);
+    $this->pluginFormManager = $this->prophesize(PluginFormManagerInterface::class);
   }
 
   /**
@@ -108,7 +108,7 @@ public function testGetUniqueMachineName() {
       ->method('getQuery')
       ->will($this->returnValue($query));
 
-    $block_form_controller = new BlockForm($this->entityManager, $this->conditionManager, $this->contextRepository, $this->language, $this->themeHandler, $this->blockFormManager->reveal());
+    $block_form_controller = new BlockForm($this->entityManager, $this->conditionManager, $this->contextRepository, $this->language, $this->themeHandler, $this->pluginFormManager->reveal());
 
     // Ensure that the block with just one other instance gets the next available
     // name suggestion.
diff --git a/core/modules/outside_in/src/Block/BlockEntityOffCanvasForm.php b/core/modules/outside_in/src/Block/BlockEntityOffCanvasForm.php
index a49df93..a77a6a6 100644
--- a/core/modules/outside_in/src/Block/BlockEntityOffCanvasForm.php
+++ b/core/modules/outside_in/src/Block/BlockEntityOffCanvasForm.php
@@ -3,6 +3,7 @@
 namespace Drupal\outside_in\Block;
 
 use Drupal\block\BlockForm;
+use Drupal\Core\Block\BlockPluginInterface;
 use Drupal\Core\Form\FormStateInterface;
 
 /**
@@ -43,4 +44,11 @@ protected function submitVisibility(array $form, FormStateInterface $form_state)
     // Intentionally empty.
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function getPluginForm(BlockPluginInterface $block) {
+    return $this->pluginFormManager->getFormObject($block, 'offcanvas', 'default');
+  }
+
 }
diff --git a/core/tests/Drupal/KernelTests/Core/Block/MultipleBlockFormTest.php b/core/tests/Drupal/KernelTests/Core/Block/MultipleBlockFormTest.php
index ad13628..117db9d 100644
--- a/core/tests/Drupal/KernelTests/Core/Block/MultipleBlockFormTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Block/MultipleBlockFormTest.php
@@ -13,7 +13,7 @@
 class MultipleBlockFormTest extends KernelTestBase {
 
   /**
-   * @var array
+   * {@inheritdoc}
    */
   public static $modules = ['system', 'block', 'block_test'];
 
@@ -23,8 +23,8 @@ class MultipleBlockFormTest extends KernelTestBase {
   public function testMultipleForms() {
     $block = \Drupal::service('plugin.manager.block')->createInstance('test_multiple_forms_block');
 
-    $form_object1 = \Drupal::service('block_form.manager')->getFormObject($block, 'default');
-    $form_object2 = \Drupal::service('block_form.manager')->getFormObject($block, 'secondary');
+    $form_object1 = \Drupal::service('plugin_form.manager')->getFormObject($block, 'default');
+    $form_object2 = \Drupal::service('plugin_form.manager')->getFormObject($block, 'secondary');
 
     // Assert that the block itself is used for the default form.
     $this->assertSame($block, $form_object1);
diff --git a/core/tests/Drupal/Tests/Core/Block/BlockFormManagerTest.php b/core/tests/Drupal/Tests/Core/Plugin/PluginFormManagerTest.php
similarity index 91%
rename from core/tests/Drupal/Tests/Core/Block/BlockFormManagerTest.php
rename to core/tests/Drupal/Tests/Core/Plugin/PluginFormManagerTest.php
index 7ea0e06..193899c 100644
--- a/core/tests/Drupal/Tests/Core/Block/BlockFormManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Plugin/PluginFormManagerTest.php
@@ -1,21 +1,21 @@
 <?php
 
-namespace Drupal\Tests\Core\Block;
+namespace Drupal\Tests\Core\Plugin;
 
 use Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException;
 use Drupal\Component\Plugin\PluginInspectionInterface;
 use Drupal\Core\DependencyInjection\ClassResolverInterface;
 use Drupal\Core\Plugin\PluginFormInterface;
 use Drupal\Core\Form\OperationAwareFormInterface;
-use Drupal\Core\Block\BlockFormManager;
+use Drupal\Core\Plugin\PluginFormManager;
 use Drupal\Tests\UnitTestCase;
 use Prophecy\Argument;
 
 /**
- * @coversDefaultClass \Drupal\Core\Block\BlockFormManager
- * @group block
+ * @coversDefaultClass \Drupal\Core\Plugin\PluginFormManager
+ * @group Plugin
  */
-class BlockFormManagerTest extends UnitTestCase {
+class PluginFormManagerTest extends UnitTestCase {
 
   /**
    * The class resolver.
@@ -27,7 +27,7 @@ class BlockFormManagerTest extends UnitTestCase {
   /**
    * The manager being tested.
    *
-   * @var \Drupal\Core\Block\BlockFormManager
+   * @var \Drupal\Core\Plugin\PluginFormManager
    */
   protected $manager;
 
@@ -38,7 +38,7 @@ protected function setUp() {
     parent::setUp();
 
     $this->classResolver = $this->prophesize(ClassResolverInterface::class);
-    $this->manager = new BlockFormManager($this->classResolver->reveal());
+    $this->manager = new PluginFormManager($this->classResolver->reveal());
   }
 
   /**
@@ -87,11 +87,11 @@ public function testGetFormObjectDefaultFallback() {
     $plugin = $this->prophesize(PluginInspectionInterface::class)->willImplement(PluginFormInterface::class);
     $plugin->getPluginDefinition()->willReturn([
       'form' => [
-        'default' => get_class($plugin->reveal()),
+        'fallback' => get_class($plugin->reveal()),
       ],
     ]);
 
-    $form_object = $this->manager->getFormObject($plugin->reveal(), 'missing');
+    $form_object = $this->manager->getFormObject($plugin->reveal(), 'missing', 'fallback');
     $this->assertSame($plugin->reveal(), $form_object);
   }