If I'm designing a new application or adding a new feature to an existing one, I'll typically use the SQL Diagram feature in SSMS to design the tables and their relationships. It works okay, but has some limitations:

1) It's tied to a single server. If I want to get those diagrams onto a new server, well, I don't know a way to do it so they typically only exist on our dev servers. It'd be great to have a tool that kept these outside of SSMS and the SQL server. It wouldn't update the database on save, but would instead batch up changes until I was ready to commit them to a server.

2) It would be great if this combined some features of multi script and sql compare, and could be applied to a defined group of servers in one go, bringing all aspects of the affected schemas on those servers into line with the diagram

3) Every time I start a new diagramming session I need to set up my own custom table format, with the fields I care about. SSMS doesn't remember this when I start another new diagram. The tool I'm envisaging would remember.

4) Adding existing tables to the diagram from the SQL database (as in the SSMS tool), but with an option to treat these as part of the diagram or as external references (our model tables can have a relationship with them, but they remain read only in terms of the diagram)