I'd be more interested in the Shadow Mario transformation everyone loves to talk about, but I guess if a Bowser clone is what it takes to get him in, then fine.
Personally right now I see Bowser Jr. as the most likely Mario newcomer, maybe outdone by Paper Mario as Sakurai loves those types of characters.
(See: Toon Link)
I really want him in, especially because I think he'd be a unique addition, but I feel like the biggest blow to his credentials was that he didn't even appear in Brawl, which focused on Sunshine where he was introduced. Man I was rooting for him so hard back then; the potential for his moveset was immense, but nope. Didn't happen.
Rosalina inclusion really hurts, as representing Galaxy could've been as simple as giving Mario the spin jump as a down-b; instead they make Rosalina the poster child. However if there is 6th Mario character, he could have a shot.
Keep the hope alive!