I know this is a copout answer, but pretty much all of them. With how little space there was in Ultimate, I feel like characters like that were not of especially high priority or strong consideration. Sure, characters like Simon or K. Rool appealed to a nostalgic sensibility but I'm referring to more out there picks like Takamaru, Mach Rider, Ayumi, whatever who would be kinda hard to justify when trying to optimize only 6 or 7 unique newcomer slots.
If we return to 12+ newcomers next game, we have more wiggle room to explore lesser expected or requested picks and pull into the less active corridors of Nintendo history. Maybe we could have brought Excitebiker into Ultimate, but whose place would that have taken? With two of the highest ballot fan requests, the two most important contemporary Nintendo characters, a third party guest and the obligatory new Pokemon you've already run out of opportunities. And maybe a retro pick could have taken Plant's place, but that was deliberately chosen to be a character everyone knows, so that easy to understand novelty might be lost the more obscure or retro you get. Bottom line, the concept of "retro" characters are in much better shape next time.