The thing is that Majora's Mask is at the most optimistic 5th in priority, and most of the others have more obvious rep choices if they decide to expand the Zelda sections.
A Link Between Worlds - we don't know a lot about the other characters here yet, but it'd be fair for Nintendo to want to showcase it.
The Wind Waker HD - Tetra, Tingle, Toon Ganondorf
Skyward Sword - Ghirahim, Demise, Impa, Groose
Ocarina of Time 3D - Sheik
Can you really see a Majora's Mask candidate beating out all of those options? It would have to be all of them, because Zelda isn't going to get two new reps in a roster Sakurai said won't be expanded drastically.
And as for Wii Fit Trainer, as surprising as she was, the fact is that she's the core rep of a title that's sold 22.67 million copies in 5 years, which is around 19.67 million copies more than Majora's Mask. In that context an argument could easily be made that she's more recognisable and iconic than "Deku Link."
A link between worlds- Possible there could be a new character, but we don't know. Pessimistically all there could be is a new Link. Toon link could be cut, but I don't personally think we will get three versions of Link if there isn't some kind uniqueness to it. (not just different weapons)
Wind Waker- We could potentially get toon zelda/Tetra or Toon Ganondorf. Probably the most likely if we go by the forbidden 7, but could as easily be dropped in favor of any of the others you listed. If we only get one zelda rep would they pick another toon character to represent a remake? or go with someone more recent.
Skyward Sword - If we go for recentness zelda wii u would be priority and gannondorf could be the main villain. Not ruling it out, but I'm not going to say skyward sword is a priority when Link doesn't have his current known looks. (he could look like this in zelda wii u)
Ocarina of time- Sheik- Okay... are you saying she counts as a newcomer? she needs to be revamped? I don't know what I'm supposed to say about sheik to be honest.
Majora- Acknowledged as a possible remake for the 3DS in the future, (despite the fact they choose a link between worlds over it... for the moment). A masked link could switch between races seen throughout the zelda series providing multiple playstyles in one character (much like pokemon trainer, and those were three new pokemon reps in one character)... so if its unique/popular enough for sakurai he'll do what he wants)
To me they are all almost as likely as the next, but I don't really care too argue. Someone will get in, the others wont. I think a Majora rep would be fun, but I really am not going to argue over which one is more deserving in the recent vs relevant vs popularity battle when two games we currently almost no info on.
Wiifit has sold a lot of games, I'll give you that. But will she by herself bring in any new players? More than a zelda rep? I doubt it. She was chosen for a unique quirky wtf character that no one expected.