I apologize if this has already been brought up, or has already been discredited, but a thought occurred to me.
Sakurai has been pretty adamant that the Wii U and 3DS versions of the games will have all the same playable characters. However, he's been pretty coy about the connectivity between the two versions beyond potentially customizable attacks. He also has to be aware that the 3DS version coming first is going to spoil the line-up of the Wii U version.
Basically, I'm wondering if one of the features of the connectivity between the two versions is related to unlocking characters. Like, you can unlock the entire roster between the two versions, but there are characters that are only unlockable in the 3DS version, and characters only unlockable in the Wii U. Linking up the two would unlock the characters in the other version once they were found in their proper version. This would simultaneously give reason to buy both versions of the game (can't have the complete experience without both), as well as keep some characters a surprise for the Wii U version (until the 3DS version is inevitably hacked and the other character files are found =P)