the only other Mario character I can think of with a reasonable chance is Bowser Jr. and honestly, I'd rather have Rosalina.
Yeah Bowser Jr was the Mario character I thought had the best chance because Sakurai usually uses character importance to their series when deciding who gets in. Since Bowser Jr was the second most important villain behind only Bowser in all the NSMB games and even Galaxy 1/2 he looked the most likely.
I imagine what killed his chances was the lack of popularity. Most of the time, the other characters that are most important to their series are also some of the most popular as well. Kirby fans wanted Meta Knight and King Dedede, Donkey Kong fans wanted Diddy, and even for someone like Wolf that many complained about when he was revealed was still quite popular and heavily requested among the actual Star Fox fanbase. Bowser Jr on the other hand has never been that loved by Mario fans and so I can see why Sakurai decided to skip him in favor of a character that isn't as important, but way more loved.