Relationship classes are a wonderful things and they are great for the editing workflow. However unless I am missing something relationship classes seem limited because they require attribute relationships.
Allow Feature to Feature relationship classes...
A feature relationship could be considered a stored "geographic" relationship relating geometries by a spatial intersected method. This allows editors instant access to related geographic features using the Identify window. I see it operating exactly like a the attribute relationship class in the window.
The Feature to Feature relationship class could also help "document" the geodatabase exposing explicit relationships as a geodatabase objects. It also could be a protection mechanism warning users/admins not to delete a feature class because it participates in this relation. Not every situation fits into a Feature Dataset, Topology or Related Table.
Essentially every feature class in a spatial database is geographically related implicitly by proximity, distance, time as long as everything is playing in the same space. Allowing users to create explicit feature relationships and storing them helps document the enterprise data store and seems to fit along with what's in the geodatabase now.