Well, I suppose this is as good a place as any to start... although I don't know if we can really discuss who is "most likely to not return" since the current roster is full of characters who many were sure weren't coming back.
Corrin - inconclusive.
I like her move set, especially the moves she uses in Fire Emblem Warriors, but I can understand why some say she should be removed though. In truth however, Fire Emblem characters in general is going to take some deep thinking, so I'm not certain...
Dark Pit - complicated.
Making Dark Pit a costume character for Pit in the same vein as Alph with Olimar might make some sense for Dark Pit.. but the idea of him using the Sacred Treasures Final Smash doesn't sit right with me, so I'm not 100% on board with the costume idea... not 100% on board with Alph still being a costume either, what with Olimar's Final Smash involving the Hocotate Freight ship; something Alph never encountered during its functional days.
Actually, the whole concept of clones in general needs to be looked at... there are advantages to their existence; there will always be similar, but not identical characters that won't work as mere costumes... cloning might be the only real option in those situations. Uh... for the record, I'm not advocating for Dark Pit's return... I do kind of agree, his addition was really pointless...
Dr. Mario - removed.
I suppose I should start by saying we definitely don't need two Marios... at least some clones like Dark Pit and Toon Link have the advantage of being completely different characters, but this is literally a second version of the same character. It would probably be best if Dr. Mario became a costume for Mario... maybe they could even use custom moves to give Mario access to the Megavitamin attack for a more authentic Dr. Mario costume.
As for your idea of retooling Mario to be based on Odyssey, I'm going to have to say no to that. While I do agree he can use some updating to reflect newer games (drop the F.L.U.D.D. for Cappy maybe?) a full revision based on one game would be very unwise for a character who is both constantly evolving and yet still hold onto his traditional skills.
Cloud - removed.
Even in the odd chance Final Fantasy 7 came to a Nintendo system, I'm not really happy how Final Fantasy on Nintendo systems has been handled over the years... I fear the series as a whole may as well be called a Sony-specific franchise by this point. There are other more Nintendo related franchises from Squeenix that deserve the spot.
Lucas - removed.
Considering how similar he is to Ness, it would just make far more sense if he was a costume for him. Save the spot for another character; perhaps even a different Earthbound character.
Mewtwo - keep.
I have to strongly disagree with your opinion here (as my avatar probably makes obvious). There are few Pokemon characters with any real degree of obvious relevance, but I'd have to say Mewtwo is one of the few who has some; he's the closest thing to a villainous Pokemon the series has--he was the main antagonist in Pokken Tournament, fyi--and one of the few uber-tier legendary characters who can work in Smash. The way I see it, the three most longstanding relevant Pokemon are Pikachu, Mewtwo and Lucario. The mascot, the villain and the hero, or for a more specific comparison: Pikachu=Megaman, Mewtwo=M. Bison (or maybe Akuma), and Lucario=Ryu... at least, that's the way I feel Game Freak regards them.
Roy - removed.
From what I keep hearing, Roy and his game of origin represent an unremarkable part of the entire Fire Emblem series, and that the only people who really want him in Smash are the people who like playing him in Smash... not really a good reason to keep him around, so I have to agree he needs to be let go. I suppose he could work as a costume for someone else like Marth or Lucina... but then his fire effects would be reduced to just visual effects... which I suppose could work?
Falco - removed.
Don't get why he keeps coming back... he's still a clone, he's still his obnoxious, arrogant self, he's still no more significant than Slippy or Peppy, and there are plenty of better Star Fox characters (Wolf? Krystal?) and blaster characters (Saki Amamiya?) that could take his place.
Jigglypuff - removed.
Was Jigglypuff ever relevant? I know it was once one of the more popular characters, but that was forever ago. Its only strengths in Smash were removed in Brawl and it's been a bottom tier character ever since. And the number of viable replacements keep getting bigger... it just makes no sense to keep it around.
Link - complicated.
Once again I must disagree with your idea of retooling a character to match a single game, especially a character that actually represents about 12 different characters. This is Smash Link we're talking about; he represents all Links, not just one of them; reducing him to one specific version of Link would be disregarding the entire history of the legendary heroes of Hyrule.
That said, Smash Link could easily be updated to include more recent games, including Breath of the Wild moves (the Remote Bombs would make a nice Custom Move). Furthermore, it would be nice to see a variety of costumes based on the various Links, rather than just one incarnation as was in previous Smash games.
As for Toon Link (I'm assuming you mean Toon Link when you say Young Link), well, that's equally complicated. On the one hand, I don't see why we need two largely similar characters (well, they're different characters, but they look and fight the same anyways) and I don't really agree that we need a Toon Link just to represent the art style. On the other hand, games like Wind Waker and Minish Cap are great and I'd love to see them get represented... honestly, I'm not entirely certain about this...
Lucina - inconclusive.
I'm not sure the idea of turning her into a costume for Marth would work... there's a few noticeable differences in the dimensions of their bodies that might make them incompatible. On the concept of character costumes however, I do believe characters like Chrom, Alm and Sigurd would make great character costumes for Marth.
As for Lucina, one idea I had was make Anna playable... having appeared in just about every game, she'd make for a good representation of the whole series. She would wield a sword as her main weapon (maybe based on her Awakening incarnation?), and like Marth would also have many different character costumes, including Lucina, Caeda and Celica. There are so many A-tier Fire Emblem characters, this would be a simple way of getting many in with minimal effort, while providing some variety between them.
Mii Fighter, Brawler and Swordfighter - removed.
Putting create-a-characters and Miis in general into Smash never appealed to me... the fact that Nintendo seems to be moving away from Miis suggests there's no point in holding onto them. Also, isn't this technically three characters, not one? I mean, they're all different after all.
Sheik - removed.
While I agree keeping Zelda's one-off alter-ego is kind of pointless at this point, I'm not sure I really want to see Impa take over her move set. I loved Impa in Hyrule Warriors, with her oversized sword and water powers... would prefer she used something like that.
Wii Fit Trainer - inconclusive.
I have mixed feelings about this one... on the one hand I kind of like her, and don't really want to see them ax entire franchises. On the other hand, her move set is weird... I know, this is Smash, weirdness is to be expected... and yet some of her moves go beyond my... what's the term? Suspension of disbelief? My ability to ignore the illogical nature of what's going on. Summoning a ball of light with Sunrise Salutation and flying with Hoola Hoops... and the fact that all her attacks are literally yoga moves... I'm just not sure about her right now...
Alright then... there's my opinions on your ideas... now here's my suggestions:
Bowser Jr. - inconclusive.
I'm gonna just come out and say it... I despise this one; he just seems like a filler character, popping up to pilot a machine that could just as easily be controlled by an AI or a generic enemy, and not very consistently I might add... he wasn't in several recent games like Mario 3D World or Odyssey. The fact that he fights with a machine--meaning we fight his machine, not him--doesn't do anything to help that feeling. I confess I do like his move set, and playing as the Koopalings... but feel like he just doesn't deserve the spot, especially for a franchise that's guaranteed to have no less than eight characters (Mario, Luigi, Peach, Bowser, plus Yoshi, Wario, DK and Diddy, which are technically Mario characters, even if Smash doesn't treat them as Mario characters).
Rosalina - inconclusive.
I actually like Rosalina and how she fights... but again, I'm not convinced such a minor character really deserves a spot over so many alternative options, especially now that we've moved away from the Galaxy series.
Charizard - removed.
I never understood the love this character got... it's a generic fire "dragon" character... and if fights like Bowser with wings... the Pokemon franchise is loaded with unique character designs, why waste the spot on something so generic?
Greninja - inconclusive.
The fact that this one was added only because Sakurai thought it looked cool has always bugged me... not saying that it isn't an interesting character, but like Charizard before it, I feel like it's fairly generic as far as Pokemon go and with its generation done and over with it may be a bit outdated by this point to keep around.
R.O.B. - inconclusive.
Maybe it's just me? But I always found it odd playing as a real-world peripheral that never appeared in any sort of actual game. Doesn't help that its entire move set is made from scratch, and not because it comes from old games with little to work with, but because it doesn't come from any game.