Characters worth keeping an eye on.
Ayumi Tachibana - Considered for Melee
BotW rep - More representation for The Legend of Zelda
Dixie Kong - Was planned for Brawl as a duo character with Diddy
Legends Pokemon Trainer - Amongst the starters being Decidueye or one of his previous evolutions
Mach Rider - Considered for Melee
Octoling - Most Splatoon content in Base game of Ultimate is from Splatoon 1 having only a few music tracks and spirits. Could be added as a means of more representation for Splatoon
Rayman - Based on trophy in Sm4sh could have likely been considered for Sm4sh
Rhythm Heaven rep - Scrapped from Sm4sh
Sonic rep - More representation for Sonic
I'm not too sure with some of them:
-Botw rep Octoling and Sonic rep: All of these franchise or game already have enough content. There's no evidence that something bigger was planned for them. Botw already have a stage, 5 music, some spirits (8 I think) and 3 mii costumes. Splatoon 2 alone already have 9 music and 7 spirits. Of course representation isn't perfect, but that's the case for pretty much every franchise in Smash. That doesn't mean that those characters have no chance, I feel like a Sonic rep is worth keeping an eye on, it's just that it doesn't really fit the theory.
-Rayman: I'm not sure if he was really ever considered, we know that the trophy is there because of Ubisoft gave Nintendo some assets.
-Ayumi: I'm not sure if she would be considered a second time considering that the translation of the remake was decided later in the development. I think the timing would be too late for Smash.
-Mach rider: Honestly not really sure how they would do a whole fighter pack with him. Retro character are harder to sell as dlc because sure you can give him a moveset and a stage, but literally every Mach rider music is already in the Mach rider medley, and there's really no potential for spirits aside from Mach rider themself (and he's already a spirit). Some other retro characters were considered in the past like Balloon fighter in Melee and Takamaru in Smash 4 and I think they all have the same problem.
For the rest, Decidueye would be worth keeping an eye on, but honestly I'd be shocked to see a second gen 7 Pokémon before a gen 8 (but to be fair gen 7 don't have a stage so who knows). Dixie could happen if they want to give Tropical freeze more content, but Sakurai seems to like to give very original concept to DLC characters so she would need to be different enough to Diddy. Maybe I'm biaised but I feel like of that list Rhythm heaven is the most likely. It has way less content than Sonic, botw or Splatoon 2 (having only 3 music and no spirits, with none of them from the last game Megamix) and it feels like there's way more potential for a big fighter pack than Mach rider.