I'm definitely getting this game then.
I'm expecting a lot of roster cuts this time around (probably not as much as Melee's) but here's my prediction for cuts:
Likely Cuts!
Wii-Fit Trainer will be cut: Reasoning- Her series hasn't been relevant since the Wii/Wii U era and nobody wanted her in Smash at all, so she's the first likely one getting the boot.
Rosalina and Luma will be cut: Reasoning- Rosalina hasn't been nearly as popular in the mainstream Mario games as of lately (she was nowhere to be found at all in Odyssey), and if the Ice Climbers are returning (which isn't likely as they only have one game and Sakurai only adds in retro characters if he knows they are getting a new game in their series (which was another reason why the Icies didn't return for 4 (on top of 3DS limitations), they would replace Rosalina and Luma (unless they add in Pauline, which would be quite a shock as a replacement (would she rep Mario or Donkey Kong's series?)
Dark Pit and Lucinia are getting cut: Reasoning for Dark Pit- Kid Icarus hasn't had a new game since Uprising and Dark Pit was a highly unpopular choice for a roster spot so he's definitely going (unless Sakurai favors him to a degree for some reason)! Reasoning for Lucina- Fire Emblem Awakening is no longer the flavor of the month as it was back when Smash 4 was coming out, and she was a direct clone of Marth, I'm expecting Corrin to return over her (and Robin, but Robin is a unique character so he's a 50/50%).
Cloud will probably be cut: Reasoning- Square and Nintendo only agreed to allow Cloud in because Sakurai was being generous at the time and since Cloud has no history on Nintendo at all (aside from FFVII almost becoming an N64 entry), like Snake, Cloud won't be back!
Toon Link will probably get cut: Reasoning- Has his series been relevant at all lately aside from getting his games ported over? (and he's still a clone, granted if Breath of the Wild Link (which is replacing Twilight Princess Link) is getting a brand new moveset, maybe having Toon Link back might not be soo bad!
Greninja will be cut: Reasoning- Gen VI has come and gone, so I'm expecting a new Pokemon rep (likely either Decidueye or Incineroar will be the one to replace Greninja, since we have yet to get a proper Ghost or Dark type fighter in Smash (the closest one being Mewtwo).
Duck Hunt will be cut: Reasoning- They were meant as a retro replacement for the Ice Climbers, and since this is going to be a true sequel, this adorable (but very much hated) duo will finally be removed!
Bowser Jr. and the Koopalings will be cut: Reasoning- Bowser Jr. almost didn't make it into Smash 4's roster and I'm expecting him and his siblings to be removed as he's hasn't been quite relevant as of late!
50/50% Cuts (and those who are likely staying)!
Mega Man and Ryu: Depends on whether or not Capcom is willing to offer another go at it, since these two are quite popular with the fans all over, I can see both of them returning! (and if Cloud gets booted, maybe we might even get another rep from both series (Zero or Proto Man for Mega Man, and either Ken or Akuma for Ryu).
Sonic and Bayonetta: Since Bayonetta 3 is Switch exclusive (and Sonic had gotten 2 games on the Switch himself being Sonic Mania and Forces), these two are definitely staying put, I cannot fathom a Smash game without Sonic in it (Bayonetta maybe, but as stated before, she's getting an exclusive game on the Switch, hopefully she gets a massive massive nerf like Meta Knight did in Smash 4).
Pac-Man: Same as Mega Man and Ryu, since Namco-Bandai was the one who leaked that they would partner up with Nintendo again on Smash 6, Pac-Man has a 50/50% chance to return or being cut!
Mewtwo: Definitely staying put (as is Lucario), Sakurai would be very stupid to remove Mewtwo a 2nd time, if he does cut him again, fans will be outraged even more than them losing Ice Climbers or Wolf!
Robin: Even though Fire Emblem Awakening is now in the past, Robin is the only unique character that I can't see going, granted if he gets cut for a different reasoning than Lucina (who is a poor girl's version of Marth) it'll be that he's no longer as relevant!
Newcomers!
Inklings (confirmed, judging by the way they fight in the trailer, this implies that they might be replacing Mega Man, if this is indeed the case, then Ryu gets cut as well.)
A character from Arms (which wouldn't be too shocking, in before Lola Pop gets a roster spot)
New Pokemon rep to replace Greninja (Decidueye or Incineroar are likely to be the choices to replace him and god if Mewtwo gets cut, then both of these guys gets a spot).
Those are pretty much my predictions!
Well, time for the inevitable counter-arguments. Ahhhh... You truly don't know how much you'll miss something until it's gone...
The Cuts:
Wii Fit Trainer: Marth was literally the same way, and she was received positively. Doubt she's getting kicked at all.
Rosalina & Luma: If that's your reasoning, then no... She was in Mario Kart 8 Delux... On top of that, she's in the new Mario Tennis game.
On a side note, Sakurai had a working version of Ice Climbers on the Wii U version of Smash, so they're most likely coming back.
Dark Pit and Lucina: I think only one of them are getting cut while the other is going to get a new moveset. It wouldn't make too much sense for them to cut potential like those two. But yeah that's just me, I don't have a real argument in their defense.
Cloud: Are we still saying that? Denial won't make that argument relevant anymore. I'm not sure if Cloud will return or not, but it needs to be said that if he doesn't, it won't be for the reasons you listed. If that "needs to be on a Nintendo console" rule really mattered at all, Sakurai's whim wouldn't have convinced anybody to let Cloud in the game.
Toon Link: No... He's definitely coming back as a clone... Him being in Smash 4 alone all but confirms that.
Greninja: That's literally no reason for him to get cut... At all...
Duck Hunt: Ooooor they'll stay because him being a retro character has no bearing on anything. Point in case: Game & Watch.
In all honesty, he should've been in the 50/50 area.
Bowser Jr.: And I'm expecting him to stay since he was a highly requested character that Sakurai managed to force in!
50/50 guys:
Robin: This is the only one I disagree with in the sense that relevance doesn't play a factor in deciding who stays and who goes. Half of the roster and their requested characters wouldn't even be here if that were really the case. Retro characters wouldn't be a thing if that were really the case. Shiek would've been gone by Brawl, let alone coming back for SSB4. Roy, Lucas, and Mewtwo would've been gone for good. Ness, Captain Falcon, Marth (who'd be replaced by Lucina), Lucario, Jigglypuff, Diddy Kong, Olimar, and several more would have been cut if relevance ever dictated who got cut and who stayed. Hell Pac-man and Cloud wouldn't even be in the game if it mattered that much. Whether Robin makes it back is up in the air, but relevance will play too little a role in that decision, I can guarantee you that.
The Newcomers:
I think Sakurai is gonna pull from the Mii DLC costumes for the New character choices.
Specifically:
King K. Rool
Geno
Lloyd (50/50)
Takamaru
Chrom (50/50. Hopefully as a companion to Robin, similar to Nana or even have him in the background and switch between him and Robin using the d-special)
Zero (50/50)
And Heihachi
But hey, that's just my two cents on the newcomer roster.