My reason for thinking that it's likely is because Smash reveal periods tend to end on a more low-key note, usually a first party (Wolf in Brawl, Duck Hunt in Smash 4, Incineroar in Smash Ultimate, Byleth for Smash Ultimate's first Fighters Pass). The one exception seems to be Bayonetta, who was revealed and released at the same time as Corrin.
When it comes to DLC, first party reveals tend to promote new, upcoming, or recent games (Piranha Plant is the only exception, and it seems like it was intended for the base game going by it being the only DLC character with a Palutena's Guidance), and there aren't a ton of recent first party releases to work with. Pokemon Sword and Shield seems like something that they'd potentially go with, considering the options, and from what little I know about the game's marketing, or its anime adaptation, Cinderace seems like the most prominent new Pokemon from that generation.
Plus, we haven't gotten a unique Zelda newcomer in 20 years (so I doubt that's happening), Sakurai seems to prefer focusing on Kirby content that he made (so Bandanna Dee, Dark Matter, etc. don't seem likely), it was a minor miracle that we got two Metroid characters in the base game, and I don't see them ending things off with a clone unless the mysterious bonus/blank space from the website files is a clone or collection of clone characters. (assuming that Octoling is the most likely option for another Splatoon character)
Maybe I'm being pessimistic, but given how previous Smash speculation periods have ended, I'm not expecting someone like Crash Bandicoot, Sora, or Master Chief.