Been registered here for awhile, usually just lurk, but first post!
While not as relevant now, in the early 90's Mario was more recognizable to kids than Mickey Mouse in America.
Click for source (#15) While I'm certain that is no longer the case today I think it shows there is huge potential in Mario among other Nintendo franchises.
The merch potential alone will be huge.
As far as attractions go though, one of the challenges I think may be the fact that many of Nintendo's prominent characters don't talk, or at least not much. Most just kind of make little noises like grunts and such. I'm not sure how that would play out in an attraction, whether they would give some of these characters voices, or if they would find a way to translate the games into a ride in a way that the characters not talking much (if at all) would be alright. I would love to see a Zelda ride, but Link (or any characters in that franchise for the matter) have never talked to my knowledge so I have a hard time imagining a ride where the main characters have no speaking role. Mario says a few words and has a dedicated voice actor so I suppose they could expand his vocabulary if need be and wouldn't be too weird. Maybe I just have no imagination! Either way if done right, and with UNI's track record as of late it would, Nintendo themed areas could be great and look beautiful.
Also, Nintendo is notorious for keeping things close to the chest. Nintendo is the only game company I can think of that has had no major leaks in regards to game development. It is very rare for things to "leak" from Nintendo. If any attractions or attraction themes were to leak it would likely be from Uni's side of things I'd imagine. There are running memes in the gaming community about Nintendo ninjas silencing anyone that dares speak of what they're up to because they have been so secretive and successful at preventing things from leaking especially in this day and age.
Can't wait to see what comes out of this!