I feel kind of sad I haven't had time to work on a Zimlet of my own; but I have some ideas I'd like to throw around to see if anybody wants to pick them up while I'm lazy.
Key Mapping Zimlet - I think we're seriously lacking in this department. I think a Zimlet that configures keyboard hooks for things.. obvious ones like up, down in listboxes, control-s to send, control-o/a to open/add.. and then maybe not so obvious ones, like invocation of other zimlets etc.. all configurable, of course.
Calendar Sync - Theres lots of Calendar programs out there..
http://slashdot.org/articles/06/04/16/1333258.shtml Maybe there could be some sort of interface (other than ical subscription -- i don't think many (any?) of them allow ical PUSHing) to synchronize the Zimbra calendar to another one..