Here's my current thoughts on likely newcomers. Smash Brawl/4 had a roster estimate of 18 newcomers in base game, quite a lot! We could get that again, assuming that they aren't gonna bring back everyone (sadly). Here is my reasoning behind ALL of them.
Bandana Waddle Dee: There's no way in my eyes that he doesn't make it in this time. He has time and time again shown he is here to stay, he is canonically Kirby's best friend (besides Gooey Ig) and has had MAJOR roles in Kirby games since Return to Dreamland. In Forgotten Land, he is literally your only ally! If he were to be ignored, something seriously wrong would be going on.
Meowscarda: I think Smash will be developed at a time where Gen 10 isn't quite ready yet, so I think they'll settle for the current one. Although I don't like Scarlet and Violet, I do like Meowscarda, and it is a very popular current gen Pokémon who is also a grass type, fufilling the role of a fully evolved grass starter in the roster of Pokémon.
King Boo: Not sure how many will agree with me here, but King Boo is a likely Mario character in my eyes. Luigi's Mansion is still going as a franchise, and King Boo has appeared many times throughout the Mario franchise, and has remained the main antagonist in Luigi's Mansion. Lots of moveset potential to be had!
Waluigi: Lol. I think they will include him just because it'd be the easiest thing in the world and it'd appease so many fans. Very famous Mario character, very requested, easy to get and creative moveset potential. If he doesn't get in, then Toad will be the one to take his place.
Sylux: Prime 4 has been stated multiple times to be about Samus and Sylux. Whether that's still true or not is up to you. But Prime 4 surely will come out at least 2 years before the next Smash game. Sylux being in Hunters, teased TWICE in previous titles to return, and directly being said to be a rival to Samus is surely something. Many people predict Raven Beak, and while Raven Beak surely has tons of potential with an arguably obvious moveset, I think they'd rather promote the current Metroid title at the time.
Shadow: Highly requested and quite expected last time. Sonic will probably get another character, me thinks. I am not sure if it'll be unique though, although I'd quite enjoy Tails. However I think they'll go with Shadow, who'd be an easy echo, and is very popular.
Octolings: Shoe-in, nothing more to say, literally if you could say a character is a lock it's this one. Whether echo or not, Octolings are a must-have.
Chibi-Robo: This is a little bias, but I do believe he has a genuine chance. There aren't many new 1st party franchises to include, and it's pretty important and regular for Smash to include many new franchises throughout Nintendo's history. Although the current status of Chibi-Robo isn't that great, he has shown extremely well moveset potential, and has around 5 titles in the past 2 decades. Chibi-Robo also received a mii costume, although not saying much, that is shown that he was considered important enough or requested enough to receive one. I don't think many people would argue or question their inclusion.
Louie: Pikmin is having a major comeback, and it may be time for the series to receive a new fighter. Louie is the secondary main character to the series, having major roles in Pikmin 2 and 3. Whether he comes back to Pikmin 4 is unknown, but wherever Olimar goes, Louie will probably follow, which makes him a great contender as a possible echo fighter. However, I argue he has the ability to be unique, but I am lowering that expectation because he'd be fine as an echo as well.
Rhyhtm Heaven character: I put Chorus Kids here because they were meant to be in Smash 4. I assume they couldn't because of technical limitations. Then Smash Ultimate came along, and I am not sure what happened, but I guess they were just low priority due to how Ultimate's characters were chosen. Rhythm Heaven is a pretty iconic Nintendo franchise still not in Smash as a playable character, I think it'd make a lot of sense.
Isaac: This one is based mostly off of rumors, but if Golden Sun were to make a comeback in some form, Isaac's one roadblock would be completely removed. Already known to be highly requested, it'd be a no brainer inclusion.
Dixie Kong: Dixie Kong is a character that has seemed obvious for a few games now, but I still believe in her inclusion. If the DK series is coming back, and if the rumors about a DK movie are true, then I feel it'd really warrant her inclusion. She also appears in the Mario Movie, which is already a huge step up!
Chosen Undead: Dark Souls is a very huge franchise for Bamco, and Elden Ring was one of the hugest games in the past few years. Elden Ring although not directly part of the Dark Souls series, was made to be an evolution to Dark Souls as a franchise, and it worked. If Bamco developed the next game, and with Elden Rings continued popularity, this would be a genuinely great move for a character for Nintendo to make.
Ryu Hayabusa (Or Warriors character): Koei Tecmo is a no brainer 3rd party to happen next, imo. They've been buddy buddy with Nintendo for a few years now. Ryu Hayabusa is their main icon, however, if they wanted to, a characters from the Warriors series would make as much of sense. Nintendo has made Koei make MULTIPLE Warriors games for them. Lu Bu would be my guess, but that opens a can of worms of arguing what is viable for Smash. But I'd love either Lu Bu or Hayabusa!
Mio: I think Noah and Mio have a good chance, but I think they may go with only Mio, she has lots of moveset potential with her rings.
Fire Emblem: By the time a new Smash comes out, Alear won't be the current protag.
Zelda: I think it's time for a new Zelda character, everyone does. Who that may be? Impa? Skull Kid? Unsure. But I can't see them skipping out on a new Zelda character again.
Weirdo character: Although I don't like following patterns, this is a constant one throughout the series. My contenders would be Excitebiker, Ayumi, or Andy from Advance Wars. Excitebiker would be to represent an extremely iconic NES franchise. Famicom Detective Club has recieved a remaster that was localized, which may finally give her more credence for Ayumi to be added. Advance Wars has made a...comeback? New remaster coming...sometime...but still gives him a reason to comeback and be added, just hard for me to imagine a PROPER moveset, but I would say the same for the other 2 here.
And...that's it, sorry for the wall of text.