These are things that are planned added 'some time in the future'. Please try Mumble out before you comment on any of the ideas below.
If you want to help out with any of the below, great! Be aware that during the alpha/beta stage the code will change quite a lot, and public CVS lags quite a few hours behind developer CVS, so you might want to drop us a line first to let us know you're working on something. That way we can leave those files alone while you work.
Multiple channels should be linkable, to allow hierarchic talk structures. The difficulty is finding a way to do this so that it feels natural for all users, and you have no cases where player A and B cause problems for player C because they are both talking to player C but can't hear each other.
Ban and banlists
There should be a variable-width ban of IP ranges. Question is; who should be allowed to add a ban? .. and who should be able to see the IP?
I'm somewhat inclined to let anyone with move/kick privileges in the root channel also be allowed to ban a user without showing the IP, and let people with write privileges in the root channel see the IP of all users as well as see/ban IP ranges.