Feel like this is as good a place as any for my own predictions.
Just have the urge to type them out.
I certainly would not be surprised to see Skull Kid, Isaac, Geno, Incineror, or Banjo. They've got the hype and the history.
Ken I've got full confidence in. After that, however, I don't think there will be any more Echo Fighters of the veteran fighters. I don't think Shadow or Dixie will get into the base game. (Much less any other suggestions out there such as Medusa, Octolings, Ninten, etc.) Echo fighters of any other newcomers I think
might be possible, but the only one I can think of is Garet or Felix.
As much as I love Minecraft, I can't see Steve being in the game, but I can easily see a stage dedicated to Minecraft as well as a stage dedicated to Tetris. These are the #1 and #2 most sold games of all time, and representation would be great. I could see both levels being fan favorites.
I still think the following characters have a decent shot (albeit, not as high as the ones mentioned above): Tails, Impa, Sora, Elma, Lloyd, Shantae, and Monster Hunter.
If I had to throw out some even rarer names? I'd go with Agumon, Spyro, and Lara Croft. Let's just say I don't think they're 0%, but I don't think they're above 10% either. They're low on the totem pole, but any could easily be a surprise inclusion. I'd say Spyro if I had to choose one of them.
If Microsoft does play ball, then I could see Master Chief as a character (or potential DLC). His name rarely comes up, but I bet people would start talking about him if Banjo suddenly appeared or Minecraft got a stage.
Lastly, I'm sure you'd probably wonder what I thought about Undertale (it's my icon after all). I'll be generous to one of my favorite games ever and give Sans a .5% chance of getting in. I love Undertale, I'd adore seeing it in the game, but I don't think it rises to the level of a playable character. As an assist trophy? I'll throw out a 1-2% I adore you Undertale! ..... but you're not getting in. :/