Since the tab character is seldom used inside tweets, rebinding the tab key to something more useful, like tab completion, would be very welcome. Tab completion for @usernames or #hashtags would be good.
There is no need to implement a full tab completing function. All one needs is a function that provides a completion table to be used by other completion libraries, such as hippie expand. Tables that come to mind include followed users, hashtags used in the past, followers, and previously mentioned users.
I can see how tab completion for usernames can be done by using twittering-get-list-subscriptions, but I'm not sure of its behavior so that's the extent of my usefulness.
Hashtags would probably be weirder. There would have to be a running list of all the hashtags you've used—that's the only way I can see it working (given my cursory knowledge of the API).
Definitely a +1 for username completion.