I've mentioned before in the main Smash Bros. thread that I'd like to see more villain characters in the game (I was so happy to see Ridley and K.Rool make the cut in Ultimate). King Boo, Skull Kid, Medusa, Porky, DJ Octavio, and Fawful would be great additions to the roster.
As for pokemon, I don't mind there being multiple reps from the series, there hundreds of the things, each with potential for unique movesets (and it beats having more FE characters). MY picks for additional Pokemon would be
-Gengar and Rhydon- both are iconic generation 1 pokemon, Gengar being a ghost type could have potential for a unique moveset, and since Rhydon was the first pokemon created (even being in art for Capsule Monsters, which later became the Pokemon series), it seems fitting that he become playable
-Necrozma- great design and moveset, and he has an antagonistic role in Ultra Sun/Ultra Moon, so he could be another addition to the "villains" side of things
-Swampert- my personal favorite in the series, I want one in because fuck it, they're awesome
Hail to the king:
One of the things I like about the series is how it took relatively obscure and forgotten characters in the west like Pit, FE characters (which looking back now was probably a huge mistake for not just smash but for FE too), Game&Watch and ROB (and I guess BanjoKazooie to a lesser degree since it always had a strong fanbase). Kind of hoping they go back to focusing on 1st or 2nd party ips or Nintendo exclusives and dig up some long dead characters to revive, like the other racers from F-Zero, Onigashima's lead duo or Ayumi Tachibana. Overall, I just dislike the popularity poll aspect of it and just want to see random characters nobody expects be thrown in with fun movesets rather than hear about how shocked everyone is that another popular IP or Master Chief made it in.
Back before Ultimate's release, one of the characters rumored to be in the game was Mach Rider, and even though he didn't make it in, if a future Smash game focused more on Nintendo characters, I hope he makes it in.