Currently custom fields are limited to certain data types, not including relations. It would be useful to be able to create a custom field that deals with relations.

An alternative way of thinking about this would be that it would be useful to be able to customise the way relations can be selected from within the content builder.

To give a specific example, we have an article type which can have many relations from a category content type. However, for business reasons these categories belong in a large, complex 5-level hierarchy where there might be categories with the same name at different levels that have different meanings.

It would be very helpful to be able to customise the editing experience for authors when picking a category by showing them an expandable tree view of the categories rather than the default combo box so they can clearly choose the categories they intend.