So based on what we know, this smash its all about popularity.
That's why they got all vets back, and why they gave us Ridley. So based on this I'm going to list a lot of characters that could happen. Not saying they would but would make sense based on what we know.
1) King K Rool. second most requested character from the looks of it. Or even any DK rep
2) Something from Pokemon. Who even knows but I'd say its pretty possible
3)Captain Toad, hes the most popular mario rep next to Waluigi and rip him.
4) Chorus Kids, not for massive popularity, but likely had data already so why not?
5) A Xenoblade rep. Popular with Sakurai and a decent fan requests. So I could see this.
6) Shovel Knight, he gained massive popularity fast and seemed to do well in the ballot.
7) Bandanna Dee, just do it already Sakurai... He has a big following and would be amazing.
8)Monster Hunter or Pheonix Wright. Both are pretty big series and requested Capcom reps so it wouldn't shock me.
9) Steve from minecraft. Yes I know that's something many dislike, but younger kids and casual fans probably gave him huge ballot support. So you never really know, its a big enough franchise.
10) Rayman, another obvious choice. Already has trophies, is a big character, pretty popular, and Ubisoft and Nintendo are really working together so this could very well happen.
That's all I can think of for now. So thoughts? Anyone I missed that had big appeal over the years or the ballot?