yeah a smash bros. game sounds awesome. The one i played before was close tot he game but long load, huge file size. Also lots of downfalls which made it horribly boring.
Be cool to give mouse no purpose, use w,a,s,d for up, left, down, right. Maybe q, e, for a, b, and r for right trigger. you know etc something simple controls but probly hard with triggers. Or try to make them hot keys but only so many hotkeys could be used so a bit confusing.
Maybe you should wait for the new smash bros. game to come out so you can base it off of that instead, characters and maps wise. My friend is a huge nerd for the game, like i'm a nerd for warcraft so he knows every character they announced so far if you want to know what they are I could ask for you, that is if you don't find it on the WORLD WIDE WEB haha.
Also if I was you i would make one custom secret character, still balanced. Because you could predict what nintendo would have for another super smash bros. i mean this new one coming out they have more non nintendo characters like sonic and some more japanese ones like marth and roy.