Oh yeah costumes would rock! Especially if you could somehow go online and design their clothes the way you want. I would give Peach orange hair and a black dress, Fox and the gang (including Krystal) with cool new suits, Samus without her helmet, Yoshi with sunglasses, and stuff like that.
Speaking of appearance, I remember somehow taliing about this great idea of making charactes look like they did in the NES, even if they never were in that system. So like you could unlock that feature just for fun and also make fighters look super muscular, really fat, mutated, big heads, or more. And how about old sound affects such as the sounds Yoshi makes in Yoshi's Island (I liked that game much better than Yoshi's Story).
If you think about it, Master Hand would be so fun to play with as long as you played with another person using Master Hand. Here's why: the controller! You can make him move around in any direction you want and do all kinds of moves to attack. It would really come in "handy!" *gets attacked by people who hate puns*
And let us not be weary in well doing: for in due season we shall reap, if we faint not.
