I would love a six player Smash Bros. game (2 GamePads, 4 other controllers). I would teach myself how to use the GamePad just in case that 5 others wanted to play, but only one of them wanted to use the GamePad or something, and I would be forced to use the other GamePad.
6 players would also be fun on online play (unless it's like Brawl where people camp for life and stall out that stupid 2 minute timer).
Also, maybe tournaments could do 3 on 3 team matches or something. Not sure how that would work out, since that sounds really chaotic to me, but it would be fun to see nonetheless.