Tools (SSMS, Agent, Profiler, Migration, etc.)
It would be much more appropriate if I was able to save my diagram to a file and just attach it to whatever database I wanted to when I wanted to make changes. It would need to fail gracefully and/or ask me if I wanted to update the diagram if the underlying objects could not be found. It would also be great if I could open the file while disconnected from a database in a readonly view or something.
Faster Development because I would not have to recreate my diagrams every time I recreate the database.