I don't like talking about who 'deserves' to be in Smash or any of that nonsense. But, I feel like Ultimate really neglected the part where it was a Nintendo game in favor of going all out with guest characters. There are tons of cool and interesting Nintendo characters who would be a lot of fun in Smash, but Ultimate really didn't have any newcomers along the lines of Little Mac or Ice Climbers.
I've been waiting for a Sin and Punishment fighter since Brawl, Saki makes most sense due to being an AT/Mii costume but my heart is with Isa. My reaction to seeing Sukapon as an assist trophy was "who is this and why isn't this a fighter." And if you want a newcomer who turns heads, how about Dorothy Gale from Kansas, now a Nintendo character thanks to Code Name Steam and the magic of public domain.
Lately I've also had a lot of appreciation for how Smash 4 dug into the Nintendo library in ways deeper than just throwing out a character. Like stages for games that don't make too much sense for a fighter (Pilotwings, Living Room), or stages representing specific stuff from the catalog of a broad fighter (Wrecking Crew, Find Mii). Then also how Villager somewhat doubled as a Balloon Fight rep with the up special, while Duck Hunt represented the NES Zapper itself.
Ultimate bringing back all of the old stages definitely had a big trade off in terms of having to make the few new stages in the base game about new characters or the hit new games. I think it was a fair trade off though, since the past games already had such a great selection. In terms of games I'd like to see get stages where it's not just an extension of wanting the character, a Steel Diver stage would be great. Snipper Clips or Pushmo could make fun stages with the environment being manipulated while you're fighting.