as for first/second party reps, aside from the original 12 we're def getting peach, bowser, wario, ridley, falco, zelda, sheik, shulk, pythra, marth, zero suit samus, pit, palutena, lucario, diddy kong, dedede, meta knight, mr. game & watch, villager, isabelle, bowser jr./koopalings, olimar, and maybe rosalina, ike, duck hunt, little mac, lucina, toon link, mewtwo, dark pit, wolf, daisy, greninja, charizard and k. rool.
Ganondorf will def return, but might technically be an entirely new fighter based on his TOTK incarnation, using his sword, naginata, kanabo and bow. he'd be a replacement for Byleth ig
as for new first/second party reps, there's def gonna be whoever the most recent FE protagonist and starter Pokemon are, and probably Raven Beak from Metroid Dread and Noah from Xenoblade 3. maybe old Rex from Future Connected, as well. Fecto Elfilis from Kirby and the Forgotten Land would be cool to give the Kirby series an actual villain rep. also we can't count out Waluigi. i'd also like Ghirahim to get promoted to playable, or see Wolf Link and Midna added as a duo fighter to replace Banjo-Kazooie. hell, even Zant would be pretty cool. i also think Krystal has a shot tbh
reps from third-party series that are already in are def gonna include Chun-Li and most likely Shadow (or Tails or Knuckles maybe even Eggman) from Sonic and Proto Man, Bass or Zero from Mega Man. Raiden and Alucard'll probably be in if Snake and Simon return, and we'll probably get Jeanne as a Bayo echo fighter. if Terry comes back we'll probably get Geese Howard or Kyo Kusanagi, M. Bison or Akuma could be added as another Street Fighter rep, and if Kazuya's back we're either getting Jin Kazama or Heihachi. another FF fighter would either be Tifa, Squall or maybe even a character from an NES/SNES FF like Cecil Harvey from FFIV or Terra Branford from FFVI. Kefka would be the best pick, though, bc he's arguably the most iconic pre-FFVII FF character. I don't see KH getting another rep even if Sora comes back, but it'd most likely be Riku or Roxas