I dont know if someone mentioned it, but for you talented folks, i think the easiest and quickest way to do it would be to remake character models to make them look like someone else and then, try to change their animation and moves.
For example, the easiest one to Edit would be Sonic to Shadow.
But again, if you are ready to really spend your free time on making a real new character and not an other melee clones, then i would like to see all the Brawl Assist trophies as a real brawl character. In faq, you can take their movement animation that is already made, their move that they use while assisting and you would only need to add hit box to them, more move so they can be complete.
Not all the assist trophies, but some of them would be more simple to edit then just making a whole new character.
http://www.youtube.com/watch?v=p3G-kTCMoAc