I try to stay out of this as much as I can. So much so that I often try to break up this flamewar by bringing up my well insightful objective reasoning on who would join that battle and why. I change my views repeatedly when circumstances change. For example, I initially thought that we'd only get 44 playable characters, and that the gematsu characters would be in, and Ike, ROB, Lucas, Wolf, Snake, Ivysaur, and Squirtle were all out. Of course, once Ike was shown, I changed my stance and brought everybody back except the latter two. Once E3 passed, however, I started taking other possibilities like Ridley and Mewtwo seriously as returning veterans. Robin and Lucina making it in with Chrom's disconfirmation, however, made me reevaluate my views altogether and now I am convinced that there are hints both in the promotional material and outside of it, on who's going to be in.
For example, for the longest time, I was speculative on Shulk's chances and simply kept him on my list of newcomers since he seems to be arguably the most highly requested character to join the battle. Once Monolith Soft retweeted the news, I knew that Shulk was in fact a playable character in this game--even when it turned out our newcomers were Robin and Lucina. Ridley is the only character that I'd give the guarantee to being playable, however, given how he is teased a ton (though if anything, we probably won't know for sure until the game comes out since it is likely that he'd be a secret character). I am also fairly certain on Dark Pit as well, and it's a safe bet that, at the very least, King K. Rool will be in the game, but I am not as sure of his chances at being a playable character.
However, some hints are not quite as cut-and-dry. For example, the Fi trophy only hints at a Skyward Sword rep, but deductive reasoning points towards Impa, Ghirahim, Demise, and Groose as the only major characters left in that game with a shot at being playable. The Sneaky Spirits only hint towards a Rhythm Heaven character in general; if you want to apply the games where they appeared, you might narrow your list down a bit, but just about anyone has as much of a shot as the Chorus Kids. The difference, however, is that one can apply deductive reasoning to narrow down the Skyward Sword rep even further, and one can deduce Ghirahim--or be wrong and just as likely get Impa. With the Sneaky Spirits, the only near-certainty is a playable Rhythm Heaven character.
Mewtwo is another story, but given the finalization of the roster began around the time that Sakurai said that he "...was thinking about..." adding him back into the roster, he seems like a safe bet, but hardly a guarantee.
That just leaves my odd prediction of an 8-Bit Mario as a playable character. I was contemplating who character 55 could be should we get that many, and I was contemplating a retro newcomer. What I wasn't paying a great deal of attention to was with the patterns with the joke characters in Melee and Brawl--specifically Mr. Game & Watch and R.O.B. respectively. What do they have in common? They are retro; they are unexpected; they are unlockable; and they are directly tied into important chapters in Nintendo--and by extension, video game--history. 8-Bit Mario fills all these roles arguably better than R.O.B. or Mr. Game & Watch. What's more, I discovered the possibility and toyed with the notion that Sakurai would even do such a thing, and over time came to enjoy it. It's not much in the way of teases, hints, evidence--take your pick--but in the amiibo segment of the Digital Event featuring the Treehouse people, all the bubbles had a character that was either confirmed, disconfirmed, or will be confirmed at a later date--including Lucina and Mr. Game & Watch. What was interesting was that Mario popped up twice--one in his 3D model, the model that we see in Smash Brothers, and another based on his appearance in Super Mario Bros. on NES.
All that was what got me to think that we'd get 8-Bit Mario as a playable character, alongside King K. Rool, Chorus Kids, Ghirahim, Ridley, Dark Pit, Mewtwo, and Shulk. There may be more outside of these eight--or even instead of these eight--but I wouldn't put it passed me to suspect that Sakurai may be acting rather coy with all eight of them. I do not know what to think of characters like Dixie Kong or Bandana Dee or even Isaac--even though I find the latter to be highly unlikely. But for all I know, somebody else completely unexpected (from my own perspective) could join the fight.
As far as returning veterans go, I am just assuming that Snake, Ivysaur, and Squirtle are the only sure cuts. Lucas is a maybe, and R.O.B. and Wolf aren't totally safe yet--but for the time being, there's no reason to assume that either of these three are going to be gone.
Now the real question is, who do I want in the game? Well, I'd take just about anybody so long as they aren't Ganondorf-levels of bad like he was in Super Smash Brothers Brawl. Other than that, I am just waiting for Meta Knight and Falco to return, and I'll be a happy camper.
Still, I'd much rather take Hades over Dark Pit. But I won't complain if I like what I see.