If the final fighter is a...
First Party Character:
I'm thinking maybe either a new Zelda or Metroid character to promote BoTW2 or Dread/Prime 4. For Zelda my guess would be either AoC Impa or one of the champions perhaps. Monk Maz Koshia would also be a super cool choice. For Metroid, I could see Sylux from Metroid Prime Hunters being a good choice for lore reasons. PLEASE no Pokemon though.
Personally, a new Kirby rep like Bandanna Waddle Dee, Marx, or Magolor or Mother rep like Giygas, Porky, or Ninten would be cool, but I know that is most definitely not happening. Qbby and Andy are other first party choices I would love, but are very likely not happening.
If Nintendo REALLY wants to pull a wildcard though, Ayumi Tachibana being the last character would actually be pretty dope.
Third Party Character:
I am guessing the following: Crash Bandicoot, Rayman, Master Chief, Ryu Hayabusa, or an SMT character. Rayman I think is long overdue and really should have been in since Smash 4 in my opinion. Ryu Hayabusa is in the same boat as Megaman and Simon Belmont in that he's been around since the NES days so I think he deserves a spot. Plus, he represents multiple genres (platforming, action adventure, fighting) so that's pretty cool. People mention Doomguy a lot, but at Bethesda is now owned by Microsoft, I see Master Chief as the more likely FPS rep. Plus, Master Chief in Smash would be a pretty surreal experience. Crash Bandicoot is just a highly requested character and is a mascot that pretty much went up against Mario and Sonic back in the PS1 days so it would be nice to see this come full circle. With the reveal of SMTV and the hype behind it, I would not be surprised if we got either Demi-fiend or the SMTV protagonist to promote SMTV.
Personally, I REALLY want Crash to be the last fighter as he is my number 1 pick, but let's see what happens. Demi-fiend or SMTV protagonist would also have me pretty hype. Otherwise, the other choices are pretty cool too, but I admit I would be a little underwhelmed if it's Rayman or Ryu. Master Chief would be a little hype just because it's Master Chief in Smash and Halo is cool so yeah. Sora is another character that I want, but not as much as I want some other characters that I have mentioned previously. That being said, I don't think he will be in because Disney and also because Sephiroth reduced his chances in my opinion, but KH fans stay strong on this one which is why I wanted to mention it.
Indie Character:
Definitely not happening, but if it does, then I see either The Knight from Hollow Knight or Reimu Hakurei from Tohou making it. Personally, I would flip **** (in a good way) if The Knight somehow got in, even as a Mii costume at this point honestly. Zagreus from Hades or Miriam from Bloodstained would also be really cool, but they would not be as likely as The Knight or Reimu.
TL;DR
First Party:
- Likely Zelda or Metroid
- If Zelda, BoTW/AoC rep (Impa, Champions, Maz Koshia) to promote BoTW2
- If Metroid, probably Sylux to promote Dread/Prime 4
- Pokemon is also possible, but PLEASE no Pokemon (at least be a dragon-type if it is)
- Wildcard pick: Ayumi Tachibana
Third Party:
- Crash - Highly requested, Mario, Sonic, Crash rivalry comes full circle
- Rayman - Should have been in Smash 4 tbh
- Ryu Hayabusa - Call back to the NES like Megaman and Simon + Koei Tecmo rep + technically another fighting game character
- Master Chief - Breaks the internet, cool FPS rep, more likely that Doomguy due to Microsoft owning Bethesda
- Demi-fiend or SMTV Protagonist - To promote SMTV
Indie:
- Extremely high chance of not happening
- If it does, either Reimu or The Knight (Hollow Knight) as those are really the biggest ones left
- Knight Mii costume would be super dope though