If Pirahna Plant comes back in the next game (which is a debate I don't want to start), we all agree that it's going to have a singing taunt right?
It's late so you're not getting any explanations out of me, just a list.
1. Bandana Dee
2. Waluigi
3. Isaac
4. Astral Chainer/Akira Howard
5. Ring Fit Adventurer
6. Gholdengo
7. Octoling (Inkling semi clone)
8. Jeanne (Bayonetta echo)
9. Papyrus
10. Dante
11. Master Chief
It is no longer late so I will explain my choices.
1-3: basic fan request choices. Those three are probably the biggest first party requests left, and two of them are also in plenty of modern games. No reason why not.
4-6: I hate the term "promo pick", it has negative connotations, but that's basically what these guys are. Two switch-era IPs getting represented in Smash, and Pokemon getting it's newest generation represented (going Ghold for basically the same reason you suggested).
7: Octoling is a collosal no brainer. My original idea was for them to be an echo, but there's enough stuff from the other Splatoon games for them to at least be Isabelle unique.
8: it's a bit weird having only one echo, but I wanted to add as many unique characters as possible and Octoling got promoted so here we are. Jeanne makes sense as a Bayo echo, I didn't have any other Sega characters lined up, and I also wanted to add more female characters.
9: I don't want to go into another Undertale tangent, I'm only a mildly broken record. You know my Undertale speal and why I think Papyrus is the best choice. If more of Deltarune is out by then, I might swap him out for Ralsei or Susie, but as it stands it's Papyrus time.
10: Dante is one of the most popular third parties. Capcom has a billion characters that should be added, and I don't even know if Dante should be first in line, but I think he's the one that would put the most smiles on people's faces. Phoenix and Hunter and Leon can all be DLC.
11. Master Chief is the big impossible character that actually wouldn't be
that difficult to add. Microsoft is already on the roster, so bringing in the Chief wouldn't be nearly as much negotiation as adding a Valve or Sony character. And even if people have started to rationalize the idea as possible, having John Halo in a Nintendo crossover would be insane. Once again, Microsoft has a billion characters that can be added as DLC (Crash, Dragonborn, Gears Man, god knows what else they'll buy), but Chief comes first unless otherwise impossible.