This would be an improvement to the display of Custom Fields.

Currently, by default (when displayed via "Automatic Display" or when inserted in an item for example as part of a "Field Group"), custom fields are rendered through the render.php layout, and you can only override the layout when you manually insert an individual field in an item (with "field 1, foo")

Currently, you can't decide a "Default Render Layout" that gets used by that field in Automatic Display or when inserted in an item as part of a Field Group.

The idea, is to add a "Default Render Layout" option here:

This would be used instead of render.php on Automatic Display and when inserting an item as part of a Field Group as a "default" option, and (like now) can still always be overriden on individual field display in an item with "field 1, foo"