Midway through Mother 3 right now and guys if you have not played this game yet play it. Now. The fan translation does it full justice - I'll go as far as to say it might be better than an official one. It's absolutely brilliant, a completely unique take on the JRPG, and each and every character is lovable, and Flint, Duster, Kumatora - they're ALL good enough characters to warrant playable status in my book. I'm disappointed with the lack of Mother 3 characters in Brawl as I'm playing through now; yeah, we got Lucas and a stage, but Porky is as much an Earthbound character as Mother 3 and no one else in the cast even got a trophy. Hopefully that'll be rectified this time around.
As for the Masked Man, while he'd bring a much needed villain rep to the Mother franchise and is awesome in his own right, my vote would go to Ninten as the next playable character... he's essentially just Ness, I know, but it seems so right to have the three boys all playable. But like Fire Emblem, all of the games have such a wonderful cast that you could probably fill a fighting game with Mother characters alone.