Geno, is my most wanted 3rd party character.
Sakurai is intrigued by Geno (look at Neito's thread.) Plus, a lot of people like him.
Sakurai has not went to SEGA (that we know of yet. No, they haven't SEGA has even said so.) Yet he is the most wanted 3rd party character.
So, i think, the two most logical 3rd party characters are Geno (because of Sakurai) and Sonic (because of us.)
Of course, the Megaman and Bomberman fans will get mad, but no matter how you slice it, somebody's pissed. And, if i had to choose, it would be Geno and Sonic because of the reasons stated above. Overall, they have the best chance to be in.
To back up the fact Geno has a better chance then Megaman/Bomberman, you see, Sakurai likes Geno. What Sakurai wants, is what Sakurai gets, so you can't really compete with him. Hey, there's always the other slot, but, the only thing the two men have riding for them is popularity, and Sonic has topped every candidate for that.