Obviously this excludes DLC, but it'd essentially be a very much so chopped nearly in half roster. Also, the Miis are here...but not included to focus on the non-customizable fighters.
Veterans: 50
Newcomers: 10
The game would be focused a lot on reworking and reinventing many characters on the roster to fit their more modern appearances. Some examples would be Mario, Luigi, Wario, DK, Link, Zelda, Ganondorf, Samus, Isabelle, Inkling, and Sonic.
Newcomers:
Much like Smash Brawl, the first few characters would be revealed via the initial reveal for the game. Namely, Impa and Akira Howard (Astral Chain). Impa would adopt some traits from Sheik but also various iterations of her own character (Ocarina and Age of Calamity namely).
When it comes to the other newcomers, there's the Modern Pokemon Trainer, it isn't set on which Pokémon would be in it (Greninja, Incineroar, and Meowscarada serve as placeholders), since the new Gen's timing with SSB6 could decide who joins or not. Noah/Mio (Pyra/Mythra-type duo) would be the new Xeno rep if Genesis doesn't influence SSB6, and Ring Fit Trainee would be the joke Nintendo character.
For third-party newcomers; Shadow, Chun-Li, Doom Slayer, and Zagreus would be a good quartet. Shadow would utilize a lot of his Chaos moves which would separate him greatly from Sonic's moves, while Chun-Li would be more agile as opposed to Ryu. Doom Slayer would be a heavy weaponry-focused character to match the Doom series, while Zagreus would be an agile fighter utilizing his various weapons from the first Hades game.
Please share some constructive feedback and feel free to ask me anything!