As long as we're dreaming... the functionality to create your own stage background using images from your Photo Channel would be entertaining...
Perhaps - through the Shop channel, maybe - programmers could make updates, including perhaps every once in a while a little update pack containing a new character and a new stage... but then again, that whole "Downloadable updates on the shop channel" thing hasn't really gone anywhere yet...
Or - and this is just a peculiar thing I thought up - a Final Battle mode in which regular attacks don't hurt your opponents at all, and the only way to score kills is with Final Smashes.
But this is all just peculiar imaginings, of course.
Of course, there is one idea so crazy that it's almost impossible to accomplish - a Monster-Rancheresque disc scan style function in which you can run any of your Wii games through the scanning process and extract the protagonist (or antagonist, I suppose) from that game to use as a fighter. Please note that I'm not pushing this forward as an actual suggestion for Brawl, but a "wouldn't it be cool if".