I mentioned in another post that it'd be neat to see a League of Legends fighting game, but that was more of a gaming crack ship. SSB opened interesting doors by taking game characters from non fighting games (and non-combat games...that random AC Villager.) and letting you play them in an all out brawl. Dissidia also let us play out our 'who would win in a fight' fantasies (ha!) for all of our favorite FF heroes and villains.
Whether it's just because it'd be cool, or if it would let you settle some old grudges, what game or series would you most love to see a fighting game of? Maybe it's even what you're working on in Mugen right now. :o
Personally, League half joke aside, I want to see a Tales faceoff game. It's practically all there already, with most of the arts resembling fighting game supers and such, you just spend more time abusing adorable critters and such rather than having a worthy opponent to parry and out combo. That and getting to punch Zelos would make the whole thing worth while by itself.