Assuming they keep everyone in (please Nintendo all I want is Corrin that's all I ask of you):
Who I want:
- Chrom (Fire Emblem)
- Waluigi (Super Mario)
- Snake (Metal Gear Solid)
- Travis Touchdown (No More Heroes)
- Crash Bandicoot (Crash)
Who I expect:
- Spring Man (Arms)
- FE 16 Protagonist (Fire Emblem)
- Wolf (Star Fox)
- Literally any new Kirby rep
- Rex (Xenoblade)
Despite the fact that apparently Chrom and Waluigi "can't join" because of Final Smash and Assist Trophy status,
other characters have joined out of statuses like these, so I feel like there's a possibility of them being implemented. I can also see Robin's Final Smash being reworked to be something involving Grima like in Codename STEAM. While I would really like to see Snake return, it's doubtful because Konami sucks. Travis Touchdown and Crash are kind of pipe dreams I guess, but considering that No More Heroes is a Nintendo exclusive franchise similar to Bayonetta and the popularity of Crash (especially after the port of N. Sane Trilogy), I'd like to believe neither are completely out of the question.
I feel like Spring Man is kind of a given because, despite the fact that it didn't really catch on, I just feel like Arms characters are perfect for Smash. Historically, Roy and Corrin were added to promote Binding Blade and Fates respectively, and given Sakurai's gracious love of Fire Emblem I don't think the FE16 Protagonist is too out of the question. With the trend of bringing back old characters like Mewtwo, Lucas, and Roy as DLC, I'd give Wolf a fair shot at joining Smash Switch's roster. I don't have much basis for a new Kirby rep, I just feel like it'll happen. I also think Rex from Xenoblade 2 will be added considering Xenoblade 2's popularity, even compared to that of the first one.
Overall, I'd like to see the entire Smash 4 roster come back, but I'd like if things were revamped and retooled, and I already think they're going in the right direction based on Breath of the Wild Link's inclusion. Personally, I think Mario and Link can stand to be reworked the most, with Mario gaining some of his abilities from Odyssey and Link gaining a completely new Breath of the Wild based moveset, giving the old one to Toon Link. Characters like Lucina, Falco, and Dr. Mario could all get some slightly different movesets rather than different damage rates on their moves as well. We're already getting Inklings, but I think their moveset might woke somewhat like Snake's did in Brawl, with our favorite squidkids using a variety of weapons and abilities outside of the Splattershot and Ink Bomb seen in the trailer. I would personally like to see their Final Smash be a cinematic Turf War or Splatfest (akin to Mega Man, Shulk, or Corrin's Final Smashes), but I can also see them making it something like the Tenta Missiles, Inkstrike, or the Kraken.
I'd also like to see a lot more Assist Trophies in the new Smash. The Champions from Breath of the Wild or a Guardian are practically shoe ins for this, and I'd personally like to see Breidablik from Fire Emblem Heroes appear as a Pokeball like device that can summon the likes of Xander, Lyn, Hector, or any other Fire Emblem character they deem popular enough to be an Assist Trophy, but not enough to join the roster. If Travis Touchdown and Crash don't make it in as playable characters, I could also feasibly see them as Assist Trophies. As for stages, either Inkopolis or any actual stage from Splatoon is pretty much confirmed. I'd also like to think that there will be a Breath of the Wild stage, with either the Ganons or Divine Beasts as stage hazards.
If they want to outdo Smash 4, I feel like they'd also add another story mode, similar to Subspace Emissary. Even though you can record video on the Switch, they could easily make it uncapturable like Fire Emblem Warriors. I know that's not stopping people with streaming setups, but I'd like to believe Sakurai changed his mind for this one. If impressions are anything, I think it's a safe bet that this Smash is going to be bigger than the last, purely because Nintendo wants to either outdo or bring over everything the Wii U once had.
Now before you call me out and say something like "Vespi you dense mother****er, all of that is too ambitious", I'm completely aware that it is, and I expect very little of it. Maybe one or two characters from my want/expect lists, but that's it. This new game has to be bursting from the seams with new content, both predictable and completely unexpected if it wants to get people more interested in it than any of the old Smash titles.
TL; DR: All I want is Marth and Corrin and we're good.