I believe we are going to receive at most five more characters in Super Smash Bros. Ultimate. I have no backup to this claim; it's just a feeling.
1.
Shadow the Hedgehog (Sonic the Hedgehog echo)
This would be an easy enough clone to make. SEGA and Nintendo are quite chummy now and it would fit in fairly well with their collaborations and exclusivity deals over the course of the past several years.
2.
Chorus Kids
Couldn't find a Chorus Kids icon, so enjoy this Sandbag icon instead! With the representation in Super Smash Bros. for the 3DS and Wii U, as well as the debate over being a cut character, I could understand if they were added as newcomers. They aren't on my list of wanted characters at all, and I believe other characters are more deserving, but I give them a good chance of appearing.
3.
Geno
There are so many reasons, I can't even begin. If you're curious, just search anything Geno related on the board... no? Okay, well, for starters, Sakurai has stated his like of Geno in the past. Additionally, not only was Geno among the characters to receive Mii costumes in the previous installment, he even got his own splash screen. The fans have been requesting him for ages. With Chrom, King K Rool and even Rathalos appearing in the next game, I believe his time has come.
4.
Isaac
Placed among the top (albeit on the low end) of most requested characters in Japan for the poll. He hasn't appeared since Brawl, but I believe the outcry for him has been heard. With so many unique attacks at his disposal, he has potential to shake up the sword-wielding crowd. He'll be upgrading from an assist trophy.
5.
Bandana Dee
One of the top picks for the poll in Japan. With Sakurai's history with the Kirby franchise, this one could be a no-brainer. That said, he's never really been a character I've wanted, and it feels almost like including a Goomba with a spear. Okay, sure he's had playable roles in the Kirby franchise, but there have also been Kirby games where you can play as practically any enemy. I wouldn't mind his inclusion, but I won't lose sleep over his exclusion.
While people often debate the inclusion of other fighters such as Isabelle, Ken and a 7th generation Pokémon, I just don't see it happening. Of course, I could be completely wrong. People who make Smash predictions are never 100% accurate. I would be surprised if even two of my predictions are a reality. I'm sure we'll get at least one character similar to Wii Fit Trainer that NO leaker or member of this community will have predicted. That said, this is a game for the fans, so we'll probably see a few more fan favorites make the cut.