With Terry from NEOGEO, makes me wonder about the final fighter. What major studios/franchises/publishers are missing? Koei Tecmo doesn't have anyone, and Capcom only has tertiary stuff such as Rathalos. So we could see something else.
In terms of genre/industry defining figures, feels like Rayman would be a good fit, but he is an assist trophy, so maybe that ship has sailed? Bomberman would be nice, but is also an assist trophy.
Spiro the Dragon and Crash Bandicoot were somewhat big in their day.
A lot of folks were claiming Ninja Gaiden's Ryu was the next character, but would Sakurai really want two character sharing the same name in? Maybe someone else from that franchise.
Moving on the the next wave, it would be great to continue the path of getting 3rd parties in, but I do hope they round out some remaining key characters from the Nintendo side.
Waluigi and (Captain) Toad would be obvious shoe-ins.
Could easily get another Pokemon from the Galar series as that seems inevitable.
I know some may not like it, but a Fire Emblem character is probably coming too for Three Houses. My hope is that it isn't just one of them, but make the professor/Byleth act like a more active Pokemon Trainer and allow him to toggle a supporting student from the thee main lords to really make it a variety pick.
Geno probably my last "niche" fanservice hope.
For a WTF-Out-Of-Left-Field pick how about Chargin' Chuck from the Super Nintendo games? We could use a more sports related entry in the roster and aside from Waluigi or another generic NES character to represent those old sports games, Chuck could be a fun one. Have him tackle opponents and throw a ball at opponents.