The babies: People tend to exclude them out of their predicted rosters. I am totally fine with excluding Baby Peach and Baby Daisy (or the ones in Mario Super Sluggers), but Baby Mario and Baby Luigi are no. Both these guys appear in a quite well-known game, Super Mario World 2 Yoshi's Island and both play a big role here. Plus, the idea of a baby version of Mario playable at the same time sounded quite unique and very weird. But Baby Luigi should be the ceiling of the addition of baby characters, since Nintendo got slightly overboard with them.
Wario: Ah, the fat man "everyone" hates. I even admit, he's underrated. Very few people has him as a favorite character, but in my honest opinion, he's a very funny character who has tons of personality and humor in him. Sure he might be a little mean, sure he might be a little stinky, but overall, he's a good, quirky, comic relief character.
Donkey Kong: Donkey Kong is a prime example of underrated. He's been around since the very beginnings of Mario (don't get this argument, "but that was Cranky Kong!! :P). Yet....he doesn't get any love. For example, I never ever hear people say, "I hope Donkey Kong returns in that game! He's my favorite character!". Donkey Kong is a cool character. Not only he sounds funny and is a funny opponent, he's fun to play as too. I played only Donkey Kong 64, but I didn't mind controlling a big furry ape at all.