I figured I'd give my input on which stages are returning and which are not
Peach's Castle and Kongo Jungle: While I know even stages from the Wii U version are getting recreated, it would still seem pointless not to just port these stages over as they'd look identical anyway. Especially since Kongo Jungle Melee was renamed. But it leaves me wondering if Peach's Castle Melee will be renamed
Zebes: It's very similar to Brinstar. While also different enough that both can coexist. But I do kinda doubt it
Sector Z: Highly doubtful, while a bit bigger than Corneria, they're still essentially the same
Mushroom Kingdom (both): I think we're getting one or none. We do already have Mushroomy Kingdom too. And Mario Maker for SMB1 stuff. But I do think we'll get one. Both is possible but not likely in my books.
Rainbow Ride: I'm leaning towards no. Stages as dynamic and platform heavy as this seem be a thing of the past. Even Orbital Gate was more like different sections instead of constant movement. And 3D Land either always has plenty of place to land or it's not actually scrolling. We also do have a lot of Mario stages already
Yoshi's Island Melee: It was in the Wii U version, but those stages are getting updated too. So just porting could mean standing out more unlike the 64 stages. But it has been here since Melee so why not?
Fountain of Dreams: It's a megaton fan favourite for its beautiful visuals. It has no reason not to return honestly
Poke Floats: This one gets mixed love and hate. But I don't think it's returning, very dynamic like Rainbow Ride and you start on a giant Squirtle who is also playable.
Brinstar Depths: I'm leaning towards yes. We know the stage is getting a new remix. And it had a very memorable atmosphere and Kraid is pretty popular. Helps Metroid only has 3 stages back so far.
Mute City: Mute City is coming back, I have no doubt on that, question is which one. I actually think it will either be both or this one. Mute City is the most iconic F-Zero location. It should come back
Icicle Mountain: Vertically scolling stages seem gone and it wasn't exactly popular
Delphino Plaza: I hope it's back. We are getting a new remix so I think it has a good chance. But we do have a lot of Mario stages
Mario Bros: This stage was a mess. 8 player might improve it but this was one of the most unplayable stages ever made, but I could see it coming back. But I do lean towards no if they have to limit themselves.
Jungle Falls: See Icicle Mountain
Pirate Ship: The only unconfirmed Zelda stage. It's a stage to me that should return but Spirit Train does seem to have the toon games covered. Especially with it being in the background of Toon Links page on the site. So I'm actually thinking its not coming back which is a shame to me
Hanenbow: This and Miiverse are currently the only emblemed universe not in smash ultimate. I think that's probably how its going to be too.
Golden Plains: This is a tricky one. We know Golden effect is back, but it's affect has been changed to that of a Xerneas like powerup rather than completely stopping knockback. It's gimmick was initially designed to take advantage of 3DS but it be tweaked to work on other systems. I lean towards no though.
Rainbow Road: This stage should be back, Rainbow Road is probably the most iconic Mario Kart track out there and it sucks we just have two Mario Circuits right now, even with so many Mario stages, this just seems like something that should be back. I mean the Wii U version had 4 different Rainbow Road themes.
Paper Mario: I think we should have a stage from the Mario RPG series. But I think we're getting a new stage instead. Mario has always gotten more than one new stage. Even with all of its returning stages I still expect Mario to get multiple new stages too. On top of New Donk, I'm thinking a Bowser themed stage (me thinks World Bowser or generic castle) and a Mario RPG stage (Paper Jam would be brilliant if pulled from the entirety of both series)
Dream Land: This stage gave me the impression it was only there for the handheld aspect of the 3DS version. It'd have a cool gimmick if it wasn't limited to one franchise. I'm leaning towards no
Mute City: While I think Melee Mute City will take priority, I actually think we're getting every F-Zero stage just like Earthbound is. It's also unique in its layout among the F-Zero stages and stages as a whole.
Pac-Maze: Here's the thing, I actually think we're only getting one stage per third party franchise, and it's gimmick was very dependent on the 3DS.
Woolly World: No way it doesn't in my opinion. Woolly World was successful enough, got a bunch of tie in amiibo, a 3DS port and a sequel on the way. With its unique aesthetic, I can't see them leaving it out
Jungle Hijinx: I'm curious how this stage will function with more dynamic backgrounds but I don't think it's a big deal. It should return
Pyrosphere: Definitely don't see this happening, with Ridley FINALLY playable, I don't think they'll also have him as a boss, even if that was a clone
Orbital Gate Assault: They put a lot of effort into this stage, I don't see them not returning it. Plus it helps it might have actually been spotted on Meta Knights stage
Gamer: I don't see why this stage wouldn't be back. Unless the dynamic backgrounds would affect the mum too much. But Wario only has one stage so far. And this was an awesome stage.
Windy Hill Zone: Again, I think Third Party stages are stuck with one stage each, sort of equal them out in way. And Green Hill is THE sonic stage. Plus if we got another, I think it would be from Forces or Mania
Miiverse: nope