As far as ideals are concerned, I'd really like to see the stage builder return in the Wii U version.
I enjoyed playing with it in Brawl, and if it returns, one can only assume at least some improvements have taken place. And if it's back it's almost certain that sharing stages online is possible. They technically did this with Brawl, it was like, players sent in their custom stages and one was sent out every day to everyone who opted into this, it was called Smash Service, I think? Something like that. It was pretty limited, obviously, but Brawl could do it. So I'm sure Smash 4 could do it on even a slightly better scale, such as at least sharing stages with friends.
As far as the truth goes, the only evidence of it returning right now is the Amazon leak. Which I'm ready to believe. But it could be false. I'm at least willing to bet that if the stage builder is back, it'll probably show up in the Direct this Thursday.