I'd really like to see Misty from Pokemon.
Like Ash, Brock, Gary, and Prof. Oak, she is also a classic, nostalgic character from the Pokemon franchise, and the only member of the "core 3" from the cartoon that has yet to make an appearance on SMB (though she has been mentioned a few times). As a female, she'd also bring some variety to the Pokegroup. Maybe even be a bit more no-nonsense/serious (but in a comedic way... if that makes any sense). Brock's a fellow gym leader, so perhaps she crosses over into the real world to find out why he's shirking his gym duties. I think it would be interesting if she were actually competent in some way, as opposed to the other Pokemon people, to sort of stand out. She could be the straight man of the group, as it were.
As for Misty's costume? It wouldn't be too hard to pull off. Here's what you'd have to work with:
Now, I think the simplest way to make this into a viable costume would be to use a sleeveless, cream yellow hoodie and some blue short shorts. The actress would then just need to tie up her hair into a side ponytail and voila!
And considering my post, it probably would go without saying, but it would be awesome if Ash returned in season 5! I think he and Brock and Gary and Oak (okay, ya, I just named off all the Pokemon people) are the funniest characters in SMB!
I'd also like to see Mega Man appear.
Mega Man is a classic video game character, and I'd say he ranks up with the likes of Mario and Link. Plus, he has the ability to copy powers and use them with his buster. Considering that season 5 is set to have a huge army and all that, I figure the more warriors that Mario can recruit, the better. Mega Man is a robot boy with a lot of power. He prefers peace over war, so he might be reluctant to join up at first... but if Mario were to convince him of what would happen if Ganondorf's army were to be victorious, Mega Man's sense of justice would surely kick in. Mega Man's costume might be tricky, though, I will admit.