Bowser jr. is the best choice for a Mario addition! (#3 on my top 5 list) Toad is too gimicky (if they do put one in, use Toadsworth), and has little to no move potentail (at least no good move potentail), and Geno is technicaly a SquareEnix character, seeing as they own the rights to him now. which would take up a third party slot. and lets face it, with third paty characters like Megaman and Simon Belmont, Geno can burn in hell for all I care (or burn in... where ever it is dolls go... satans toy box..). but I do agree that shadow Mario shouldn't be a transfomation for him.
Bowser jr. would definatly have an original move set useing the paint brush. heck, he could probably be original even without it. people have already givin' perfectly good move sets for him already, so I'm not going to bother with one unless absolutly nessesary.
for the people who dont want him in. give an acual reason why he shouldnt be added. all you seem to be saying is "omg! he replaced the koopalings!" well guess what, the koopalings were in Mario&Luigi: superstar saga. which isnt that old I might add. the koopalings were added ONLY for extra bosses. which is as gimmiky as it gets. and the fact that there are too many Mario characters already... what did you expect!? it friggin' MARIO! of course theres going to be a lot of characters from the most well known and best selling video game franchise of all time. there arn;t even that many Mario characters that deserve to be in anyways. the amount of characters in Brawl from the Mario frachise, would be no more then Mario, Luigi, Peach, Bowser, Bowser jr, and (maybe) Toadsworth. which is only 1 more additon then melee (with the removel of Dr. Mario) one character addition is nothing to get your panties in a bunch over.