I could very easily be wrong, but I have a strong feeling these four are coming.
I'm a bit biased here, but without talking about if they're likely or not, I honestly believe that these 4 would please most of the community, or at least that's my feeling.
- Geno and Crash while not being first party, are two options that most of Nintendo fans would enjoy. Geno is obviously a long requested character that would appeal to the core fans. His Mii Costume hasn't appeared yet, which also helps him. Of course, the costume could return with FP7 just like happened with Heihachi, but so far it hasn't and that's a good sign for him. Then there's Crash, which is a really popular character in the industry since the 90s and has seen a strong comeback in the recent years. Besides, even if his popularity within Smash fans is relatively new, he already seems to be a really popular suggestion among the community.
- Dante and Chief would mostly please people who wants some third parties, and these two, especially Chief, would make a lot of buzz with their addition to Smash. Both are iconic and popular options, besides being influential in different ways: it may seems a bit obnoxious to say, but it's not stupid to say that Devil May Cry basically created a sub-genre within action/adventure games. Dante's now more popular than ever thanks to DMC5 as well. Then there's Chief and Halo. Halo: Combat Evolved, the first game, is usually considered the game that popularized playing online on consoles. Obviously that's not the only legacy that Halo has, because it's the flagship IP of Xbox and at least during the decade of 2000 Halo was one of the big FPS franchises out there. And I could be wrong, but I really believe that Chief would be considerably more groundbreaking than any character added before.
Obviously, other characters could do a similar job. It's not like it's exclusive to these four names, but the case is that I feel that these 4 could make a good balance in terms of pleasing different fans and tastes within the community. Geno and Crash fit more the cartoony and mascot side of Smash and Nintendo, while Master Chief and Dante would obviously fall into the 'realistic' side of characters of the game.
Also, they're from completely different genres, which I think is a nice bonus. RPG, platformer, hack and slash and FPS.
Just my opinion, of course.