I still think the most likely is the 3DS/WiiU connectivity. It's the only way where both games will get the same roster. It's like the Pokemon games, they have the same pokedex, but some Pokemon unobtainable in 1 version, but obtainable in the other, etc. It's a good incentive for "true" fans who buys both versions, which is what Nintendo wants. And it may boost WiiU sales too, especially in Japan, since most of them have a 3DS but not a WiiU.
Then again, but thankful if we have the big 4/5 at least, Sakurai could just stop with this roster, but if the connectivity is true, that means he's making more effort to create more characters for us. Don't forget that he started on Smash4 IMMEDIATELY after completing KI:U, and he still has the arm injury.