I've got a couple options:
Gengar: /v/ still has me Gengarpilled from all the fake leaks about it during Ultimate's speculation period. The fact the thing is still one of the most marketed and popular Pokémon to this day also helps.
Regigigas:
https://youtu.be/Mze6Hj7Uprs?feature=shared
Rivals of Aether's mod lives rent free in my head.
Magnemite: The fact this thing had a supposed Ballot presence in Japan makes me laugh and kinda want to see it being added since the Ballot did influence a number of fighters in Ultimate.
Great Tusk:
https://youtu.be/cWeytwy6sis?feature=shared
Don't really care about this Pokémon in general, but seeing it in action in this Pokémon fighting fangame won me over even if I know it will never happen.
Bidoof: I feel Bidoof would be a perfect Piranha Plant type pick and I would love to see a trailer using the same cinematic style as Bidoof's Big Stand.
Bloodmoon Ursaluna: No real reason other than it immediately becoming one of my favorites after being leaked. Kind of like
And finally a more unconventional one because Trainers are typically ignored in Smash outside of
but I feel this one is a potential darkhorse given her sheer popularity.
Using Haunter almost exclusively, but able to call upon other Pokémon to assist like a Mahvel style such as Banette, Litwick, or Kadabra (to double as acknowledgement Kadabra has been "unbanned" in media).
I got plenty of other ideas, but this post is too long already.