Well, I've decided to go ahead and make an incredibly optimistic roster that isn't likely to happen but is still technically feasible. So, here it is, a 57 (55 if you don't count the separate Mii Fighters) character roster.
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Explanations
Paper Mario: My ideal choice for a sixth Mario character, if we were to get one (and no, I don't care that he's basically Mario). He has a lot of unique potential and could offer a lot as a representative of his own mini-series within the Mario universe. With a Paper Mario stage present on the 3DS, he may not be out of the question.
King K. Rool + Dixie Kong: I really only expect one Donkey Kong newcomer, but since this roster is optimistic, I decided to include both. I don't think these two require explanation.
Tetra: Seems like no one remembers Tetra nowadays... At this point, I think she's the most ideal choice for a sixth Zelda character, as odd as that may sound (and again, I don't care that she's technically another Zelda: she's nothing like the two forms of Zelda we already have). Basically, she's important (essentially being Toon Zelda), she's even more recent than someone like Ghirahim due to Wind Waker HD, and she has unique potential. She's also another character from the "toon" games, meaning Toon Link wouldn't be the only "toon" character on the roster. Maybe it's an unpopular opinion, but I think Tetra would be a pretty good addition to Smash.
Ridley: Self explanatory.
Mewtwo: Pretty much the most popular character request, has every reason to be in Smash, slots don't matter, etc.
Shulk: Good character from a great game. He seems pretty likely.
Rhythm Heaven Character: The Rhythm Heaven girl is just a placeholder for any Rhythm Heaven character. I think some sort of character from that series is fairly likely due to the Sneaky Spirit in Smash Run, but who it may actually be is up in the air. Could be Chorus Men, could be Marshall, could be Karate Joe... I don't know who it will be, but I think we will get someone.
So, thoughts?