I highly doubt they'd take Shulk out... he is the original Xenoblade character after all, and they haven't replaced Marth yet and he's from a series with way more options, including plenty of viable replacements. Rex seems a likely candidate, although I still wish some of the secondary characters could get some love... Fiora and Morag have such uniqueness to them, they'd totally make great and original fighters. There's Elma too... but I can't help but feel like she wouldn't bring much originality to Smash.
Splatoon and the Inklings seem an obvious addition... too successful not to add.
Don't know about ARMS... maybe... not sure one reasonably successful game makes a franchise worth representing. I just really hope they wouldn't add Spring Man... do we really need another Little Mac character, but with slinky arms? Would much prefer Min Min... interesting weapon choices (laser breathing dragon) and kungfu moves.
Would love to believe Dillion from Rolling Western has earned a spot, what with a third game on the horizon.
Still plenty of old games to fill the 'retro' slot... really thinking Takamaru and his samurai skills could take that spot, especially since Sakurai seemed to considered him for Smash 4, but decided against it because 'his game is exclusive to Japan' as if that really makes any sense...
As for third-party characters... I'm still thinking Shantae has earned a chance. She's four games in, all on Nintendo platforms, has global recognized now that her games are released in Japan, and arguably the biggest and best representative of some of Nintendo's biggest supporters; the indie developers.
Not sure if they earned it, but I'd love the Wonderful 101 to be playable... but only if they feature some sort of team mechanic like they did in their game; I don't want to be playing as 'Wonder Red and his amazing punching powers' as many people seem to desire, I want the whole team together!
That's what I'm thinking right now, although I have a fair share of concerns right now, my biggest ones being:
1. Will they finally cut out the old and redundant characters to make room for newer and better options?
-Jigglypuff is still in why now? Especially now that it has literally nothing going for it... is it just there because "tradition" now?
-Why do we need Falco? Wouldn't someone like Saki do the jumpy, blaster character better and represent more than just an obnoxious sidekick (like an entire franchise)? Not to mention he has better options, what with his laser sword.
-Do we really need Lucas as his own character when character costumes are a thing? Why not make Lucas a costume for Ness and add a different Earthbound character? And that kinda goes for any character who is basically a clone that could work as a costume too.
2. Will they just fill the roster with even more random characters from top-tier franchises?
-Do we still need Sheik? Zelda's one-off alter ego, especially now that she's an entirely separate character? Can't they add a different Zelda character like Impa or Tingle?
-Did we really need filler character Bowser Jr. and 'yeah he looks cool, let's add him' Greninja? Is it too much to ask that a character has some strong purpose or decent amount of consideration to be included?
-What's the point of bring Roy back? He was still a clone and worse he was paid DLC... would have made Corrin (the sixth Fire Emblem character and also paid DLC) a lot more tolerable too...
-Cloud... seriously, let's just file this one under 'jumping the shark'.
3. They're never going to update the characters that need a new move set.
-Ganondorf... should be obvious...
-Wario... deserves Wario Land moves, not that made up WarioWare set...