ok so I know y'all hate theories but hear me out on this one I've been thinking of for a while:
The "In Universe Teases" Theory
Ergo, a character's source game making a tease or hint at them being in Smash Bros (meaning company rep statements like Pete Hines's "Oh yeah" don't count, it has to be dialogue or text IN a game). Spirits do NOT count as tease results as it can be assumed they were planned well before DLC planning began, but Assist Trophies, Mii Costumes, and of course playable characters DO count.
Realized Teases:
- In Persona 5 (2017), Futaba makes a comment at one point like, "This is just like... what's the name of the game? Super Bash Brothers?" One year later, Joker is revealed as Smash Ultimate's first character of the DLC Fighter's Pass.
Rodin (Assist Trophy) - in the Switch port of (I forget if it was Bayonetta 1 or 2), there's a letter from Rodin basically inquiring about a tournament Bayonetta's going to (obviously alluding to Smash) and commenting that maybe he should come along sometime. Later, Rodin is revealed as an Assist Trophy.
Sans (Mii Costume) - in Deltarune, near the end of Chapter 1, Toriel reminisces about Kris and Asriel when they were young, saying the two loved to play a game called "Super Smashing Fighters" and Asriel loved the dino character (a reference to Yoshi). In Smash Ultimate's flavor text proper, there's also a description for one of Toon Link's moves that mentioned "dunked (on)" and "gonna have a bad time" which are dialogue associated with the fight against Sans in Undertale if you're playing the Genocide route. Fast forward to September 4, 2019, and Sans is a Mii Costume, and the only one so far bundled with a standalone music track (a Smash remix of MEGALOVANIA).
Unrealized Teases:
Shantae - in a trailer for one of the new modes of Half Genie Hero Ultimate Edition, Shantae says dialogue along the lines of "I'm gonna have the ULTIMATE slumber party, and EVERYONE'S invited!" This is quite obviously a reference to Smash Ultimate's debut, down to the "EVERYONE IS HERE!" This one could be coincidence, but with how fervently Shantae's been requested, I doubt it.
Bandana Waddle Dee - a flavor text for Kirby Star Allies if you pause when Bandana Dee is the controlled character says something along the lines of "claiming a prize for perfect attendance" and "training somewhere".
Elma - once you unlock Elma in the Land of Challenges in Xenoblade 2 (if you've purchased the Expansion Pass) by defeating her in the challenge "Otherworldly Fighter", the great Nopon Sage says something like "Is true! Is dream of everyone to see Shulk and Elma in same place together!" Followed by Nia asking, "Who the heck is 'everyone' in that sentence?" Once again a reference (slightly more subtle) to the Smash Ultimate debut trailer. The sentence doesn't mention Rex, but Rex has a Mii Costume anyway. Again though, that's just Sakurai deciding to pay Rex tribute as he didn't make the roster.
The above three do have Spirits in Ultimate, but again, I am NOT counting Spirits as the end result of these "teases". If I am missing any let me know, but I believe these in universe teases will ultimately result in something. Shantae could become a Mii Costume, but I doubt any of them would become an Assist Trophy because as far as I know, DLC Assist Trophies aren't a thing and likely won't be.