PK identifier capability
Submitted by mgress
This provides a visual (somewhat) for what I'm getting at: http://img819.imageshack.us/img819/7163/modelhi.jpg
The big thing would be the ability to make identifiers bolded(to identify primary or foreign keys) while leaving attributes unbolded.
This would open up the door for the ability to do entity clustering and supertype/subtype hierarchy such as this: