I think it would be cool if you could use your 3DS as a controller for playing on Wii U. That needs to happen because Wiimotes are weird and I forget if the Wii U still has ports for Gamecube controllers.

As for the release date, I think it is still undetermined and-wait...

Sorry, just realized the newest Professor Layton game has been out for about a week. YEEEEEEES

Anyway, I would expect it later in the year around fall. Or maybe summer. Either way I would imagine the release to be months away.

Cross-platform play won't be possible, I believe. You can expect a release date anywhere from October to December, although don't be surprised if Smash is delayed until 2015.