The new Super Smash Bros. will be coming to both Nintendo 3DS and Wii U, but fundamental differences in level design between the two versions mean cross-platform play will not be possible despite what many fans were originally hoping. That means players on the Wii U won't be able to lay the smash on those with the 3DS version and vice versa. Director Masahiro Sakurai himself confirmed the unfortunate news in an interview with Joystiq, but pledges that connectivity features are still planned between the two platforms as first promised back in 2011.
Sakurai brought up character transfers as an example; customize a brawler on one platform and transfer him/her to the other. He couldn't elaborate on what exactly will be customizable for each character, but doesn't believe costumes will factor largely into it and it won't be something requiring "boring, repetitive tasks" to unlock. Sakurai also hopes to make connectivity between the two platforms as seamless and convenient as possible.