A wtf character is guaranteed to be an important piece of nintendo history, like G&W. Third party characters aren't important enough to smash bros. to be the final unlockables.
Yeah, since this is supposed to be a kind of Nintendo all-star brawl they are almost guaranteed to make a Nintendo character the final unlockable. I mean, why make a Nintendo game about their biggest names battling it out and then the final thing you are rewarded with is... a Sega character. No, it makes no sense. The final character SHOULD be a Nintendo character (because of what I just said), SHOULD be a WANTED character (because who wants the last character to suck), SHOULD be generally a good guy (because Nintendo is like that) , SHOULD be old school (SSB: Ness and SSBM: Mr. G&W), SHOULD be new to Smash Bros. (again, already said), SHOULD be, for the most part, a surprise (this is difficult considering I said he/she should be a "wanted character"), and MIGHT fit in the least, considering the other characters for the game. For example, Ness fit into SSB the least, considering the other 11. He was from a type of game unlike the rest and had mostly mental/magical powers, whereas the rest used physical. In SSBM everyone was 3D (except G&W) was colored (except G&W), and was expected (to some degree) to be in the game. I'll admit Roy and Marth were a huge surprise, but there were, after all, 2 Fire Emblem characters and only 1 G&W.
All of that said, my money is on Lil' Mac. He's Nintendo, he's wanted, he's an old school good guy, he'd be new, he'd be (to some degree) a surprise, and he wouldn't fit in as much as many of the other characters (being a 1-on-1 type guy), plus it'd be cool.