I voted for the Belmont clan from Castlevania, with different outfits being the different Belmonts like how Bowser Jr's alts are the Koopalings. I made the points that a whip user would be a new fighting style instead of just a reskin since we have enough clones already. The sub weapons could be special moves, the Final Smash could be Grand Cross, etc. They could even have a new stage based on Dracula's throne room and the final battle from Rondo Of Blood as seen in Dracula X Chronicles on PSP, where it changes and disintegrates over time as you fight Dracula. You could even have Dracula's various forms show up like the Yellow Devil. There's not a single part of this that is a bad idea, surely? Hell, have one of the costumes be an old school 8 Bit Simon Belmont in the classic orange pixels and people will officially flip their shit.
The wife wanted the Hunter from the Monster Hunter games, which she argues could work since MH4U is 3DS exclusive now, made by Capcom who have already allowed Mega Man into the game as well as th DLC for MH4U so the link is there, however tenuous it is for now. Different costumes could be male and female hunters in the iconic armour sets like how Robin and Wii Fit Trainer have two forms, the wide array of weapons in the games would allow for plenty of varied attacks that could use different weapons from the same class for alt attacks. Example, if the Side Special was a heavy chargable attack with a Great Sword, the alts could be different Great Swords that act differently, charge faster or deal more damage, etc. The Final Smash could be a Rathalos sweeping across the screen or something. You could even make them do the traditional 'item use' pose when they regain health with food items or heart containers to keep the tactical feel of the Monster Hunter series, having to pick your moments to heal. I think it's a great idea and though I see the Belmonts as far more likely, I hope both come to fruition.
Also, to the wealth of people who keep asking for Goku, there are already about a million custom Mii brawlers for every form since apparently people have no imagination whatsoever. He's basically already in the game, so don't waste your votes.