I think Wii U will be more popular competitively because of the controller scheme. A controller feels much better than a handheld for a fighting game and replacing a controller is MUCH cheaper than fixing a handheld. Plus you can more easily stream the console version and thus building hype. Just a side opinion here but if they released a special edition of smash bros 4 with a GC controller Wii U pro controller then I would immediately drop whatever money they want for it. GC controller just feels too good for smash bros.
Oh and this is all assuming Smash 4 has enough competitiveness to be played that way.
Tourney rules, as neutral as possible stages. Meaning no walking off to your death, no moving stages where the camera can kill you, no stage hazards, and so on. No items and probably a 3-4 stock 8 min game. 2/3 and 3/5 pretty standard stuff.
Not sure who in the pro scene will return, but honestly I am more concerned about the game having competitive value first. I'll worry about other players later. I just want to be able to have a new game to play. Melee > any future smash bros game but the same can be said about MvC2 > all other MvC games. But UMVC3 is the thing played at EVO. I want smash to be able to move on like all other franchises.
If you mean characters, Jiggs, Ice climbers, captain falcon are all pretty much guaranteed or very close to it. There might be a second fire emblem character and maybe a Xenoblade character(though this one is iffy because Xenoblade is kinda new for Nintendo). Ness probably will be back too.
I'm just taking a wait and see approach to smash 4. I'll get the game yes, but how long I play it will be determined by if it has enough competitive value. I used to play ganondorf and roy in melee + a little bit of marth and capt falcon so I'll check out the fire emblem character(s), ganondorf(if he is still there), and good old captain falcon. I hope falcon has a good knee of justice.