Geno really has nothing to do with any other Square Enix character, and the comparison isn't fair to either. The IP tangle SMRPG's in is a real shame, and I wish Nintendo could just work out a deal with Square to get their hands on the rights.
I like M&L and Paper Mario, or at least I used to, but SMRPG really is something special in itself, and I'd rather have that specific thing than a substitute. I wouldn't put it up with DQV, FFVI or Chrono Trigger, but I do genuinely like SMRPG better than Earthbound (!), FFIV or DQVI. I like SMRPG more as a game than I care about Geno specifically as a character (I just like Peach and Bowser too much to use the new guys very often), but I get why people like him.
Man, I fired up my Wii the other day so I could play Dragon Quest Swords finally (and probably Metroid Prime again...), and there was SMRPG on the dash, between Mega Man 9 and Rondo of Blood.
Okay can we stop saying that "Erdrick couldn't have these stats".
Erdrick has never been playable in an action game, he isn't playable in DQ Heroes. Any Smash interpretation of him would be a totally fresh take, there's nothing to go on how he "would" play.
The new hero characters from, uh, Heroes aren't a bad place to start for a rough idea. They do represent the same sort of archetype as Erdrick, though they're missing a bunch of spells. I wouldn't be surprised to see an attack or two from Heroes sneak into Smash.