Think about it :
The Wii-U version need to sell alot. The Wii-U need it , and Nintendo know about it.
Why would Nintendo let it cool now?
The 3DS version in my opinion is not the final version , just like Mario Kart 8. And the Wii-U version will have more Characters , conecting the 2 versions will complete it.
The Nintendo Wii-U version having more characters is not bad at all , they can update the 3DS version after that.
Just think about it! The Wii-U version is in NEED of something to make it great and diferent. Having more Characters , adventure mode , more stages , etc.
What do you think?
( Sorry for my bad english )
I don't think it'll have more characters as they have said that it'll be the same roster across both. It would be a move not a lot of 3ds fans would like if they had to buy the Wii U and the game to get the rest of the characters.
Adventure mode likely isn't going to change, how much more content would possibly be worth it unless you are a superfan? You already played the game, got all the characters, and can play the main attraction of the game which is playing with your friends no matter how you play. I know several of my casual smash friends don't care at all for the Wii U version because they can already get the 3ds version which is cheaper and still get the full experience(for them).
To some this is a 60$ upgrade but to others it is 360$ and I still cannot see anybody dropping that much for just smash if they haven't gotten a Wii U already due to the games that are already out on it or will be out.
I do think that there is a small chance they'll put in more competitive mechanics and make the game more competitive friendly in the Wii U version with possible re balancing of what is found to be too strong or unfairly glitchy in the 3DS version(Rush cancelling out of light hitstun is one that I think shouldn't happen regardless of who you are). If they simply added at least one of these two things, dash dancing and air momentum carrying over from run and jump properly then I would consider actually buying multiple copies for the smash scene I organize at my college. And I'm sure other competitive groups would consider doing so as well.
And I say this because most Japanese fighting games come out in arcades first to players so they can find issues and balance and such.
I think some people might be mad at me but honestly I'm just trying to look at it rationally. Your average parent or averege Joe won't buy a whole new system or the same game twice but in HD less than half a year after the 3ds version releases that is cheaper. People who stick it out for the Wii U version likely already had one. It is going to take a TON of convincing to get people to buy it again, and even then some people won't because they already have the game.