1. Okay. You can add additional functionality using Zimlets.
2. Skins are customizable. The Zimbra folks have been working on a rewrite for 5.0.
3. Possible via Zimlets and Dwt.
4. Also possible via Zimlets and Dwt.
5. A Custom JSP makes this possible.
6. You can cross mount folders with permission restrictions in Zimbra.
7. That's a different system altogether. You would have to write a seperate applciation for that.
For 1,3,4,5, you don't need to rewrite anything, you can just add functionality to existing dialog boxes. 2 should be possible. The only one that looks really scary is number 7.