Updated: Comment #N
As we convert forms to use FormInterface, we come across cases where the form is used not as the route controller directly, and drupal_get_form() must be used.
We end up with code like this:
Make drupal_get_form() smart enough to take a class name, and instantiate it. Follow the same code flow as HtmlFormController (the class that process _form controllers):
User interface changes
API addition: Allow class names to be passed to drupal_get_form().