I've already learned from the speculation of the last three years that expecting anything is extremely foolish, and a good way to get yourself disappointed.
Honestly, at the end of the day, Mewtwo as a DLC opens a lot of doors.
Not just to more DLC characters, but to modders using those codes to add modded in characters (basically what people have done with Brawl [and adding up to 100 different characters now], only quicker and easier and already in the coding).
I think there's gonna be a lot more high quality modded characters in Smash WiiU in say 3 years than there are in Brawl now. Currently the only "high quality" characters (as in you can't really tell they're not in the vanilla game) I can think of in Brawl are Roy and Mewtwo in PM, and maybe like 1 or 2 others (mostly clones of characters in Brawl). That's it.
But for Smash WiiU, the modding will be easier given the experience of Brawl people have as well as the DLC coding and whatnot, and honestly, the demand will be higher. Given that, there's no way, at the very least, K. Rool isn't some high quality fan made character several years down the line. That's a pretty good "worst case scenario" if you ask me.
Such is the (fortunate) case in "the era of the Smash mods".