With so many characters and companies being disconfirmed, the last character is going to be Elma because of the XCX music royalties issue that held Hero/Dragon Quest back from being basegame. Fits the Vifram figure that we still don't know what it was meant for, fits the timeline as Elma was almost-definitely in contention for the promo character spot that Corrin took in SM4SH, fits the cryptic references in her XC2 DLC about "everybody finally being together", fits Thank You Theory as a means of thanking Monolith Soft for all the support they lent towards both SM4SH and Ultimate's development, fits the recent discovery that XCX was in the game and intentionally removed from the files like those 2 Rare games at some point, fits the timeline for both Sawano's secret music project from last year that hasn't been revealed yet and Caitlin Glass's (Elma's VA) secret project that's under NDA that we don't know what it is yet.
Elma was intended to be in the basegame and either cut entirely or pushed back to DLC, that's my speculation.