That's what I was thinking. The Zelda U/NX character designer(s) could easily whip up a Sheik design for the Smash 5 crew to use, so Sheik won't be an issue. It's Zelda & Ganondorf who I'm concerned about. If both aren't in Zelda U/NX or if they don't have battle-ready designs, then that could be a problem.If both Zelda and Ganondorf are in this game, I think they'll update all 3.
If not....dunno.