Chill night so I'm going to info dump my DLC predictions. Outside of the Arms rep, none of these are in order.
Fighter 1: Spring Man/Min Min
Stage: Sky Arena - Fans in the audience wear hats based on the characters chosen; Hedlock can appear as an item
We know it's arms, nothing much to go over here that hasn't been said
Fighter 2: Crash Bandicoot
Stage: Time Twister - stage occasionally warps to different areas based on popular crash levels
In terms of third parties, I think this is one of the more likely ones. The resurgence crash has had recently thanks to the remakes, the fact this means the classic titles have been on switch for some time now, the iconic status of crash as another mascot rival to Mario... this would just be an excellent addition.
Fighter 3: Elma/Rex
Stage: If Elma, Noctilum? Sylvalum?; If Rex, Mor Ardain - whichever, I would assume another moving level that showcases different levels of these huge areas
I am among those that feel that Rex and Arms being called out as not being ready soon enough for base game inclusion was a nice callout, and now that this is being remedied with Arms it would be weird for Nintendo and Sakurai to NOT capitalize on that to bring in some love to Xenoblade. I would love for it to be Elma as X was so underrated and overlooked and it would be sick to find out that one is also being remade. However, Rex also merits it at this point since otherwise he just feels like this game's version of Smash 4's Chrom
Fighter 4: Gen 8 Pokemon - Dragapult/Toxtricity
Stage: Wild Area - weather changes and different pokemon appear in the background depending on the weather. Sometimes the sky goes pink and a gygantamax pokemon appears as a hazard
I'm going wild on this one and say none of the starters will get the spot. So far the most attention seems to have gone to Raboot in the anime which makes it seem like the scorbunny line is the favored one, but because of the pokemon fan poll not giving any starter an edge over other pokemon I wonder if they'll give other ones the edge.
The other reason I would opt for these two has to do with what they can represent as something new for Pokemon. Dragapult could represent pseudolegendaries, would two super popular types still unused in smash ( ghost and dragon ), and it's probably Leon's most notable non-starter, non-charizard Pokemon.
Toxtricity is one I find less interesting, but it would be cool if they took advantage of it to get a moveset revolving around status conditions such as paralysis and poison. Mostly just mentioning it since it was also fairly popular in the fan votes.
Of note is that I don't see this necessarily happening this year to hype the DLC, but later to have a tie-in with the next game ("buy DPP game within the first three months and get a free, otherwise unobtainable dreepy!")
Fighter 5: Controversy! Also known as mobile game rep
Stage: No idea here
This is where my list gets less biased as I have no personal investment in this, but it seems logical to me. Mobile games represent a huge untapped market that could potentially expand the smash audience and also boost sales for whatever rep's game gets chosen.
It kind of amuses me that I see disdain towards Euden from Dragalia Lost because of it being mobile, but honestly that's small potatoes to the really gacha games that exist out there. Nintendo mobile games as super F2P friendly in general, and in this sense I think Euden is the most inoffensive option possible. It would also rep a new Nintendo property that has done rather well, and giving it a smash rep could boost its numbers which is a good business move for Nintendo.
On the less innocent side, I can also see this go into third party territory, with something from Monster Striker or Puzzles & Dragons, two extremely successful franchises that originated in Japan and that have already been featured in Nintendo systems (3DS). I see puzzles & dragons being the most likely one due to having so many cross-overs already and being from a more game-focused company (GungHo, also known for their upcoming Ninjala!).
On a side note, given the playstyle of puzzles & dragons, this would also bring in some puzzle game representation! Honestly, I'd rather get something with history like Lip or Arle, buuuuut... money is money and I think business-wise this is a huge crossover, particularly in Japan, that people are sleeping on. Mobile reps in general are being slept on super hard.
Fighter 6: Waluigi.... or what if a mario RPG rep?
Stage: Marina Stadium (Mario Tennis Aces) if Waluigi; ??? if mario RPG rep
Given the fuss waluigi brought about after being confirmed an assist trophy, I could see Nintendo aiming to go for this. It would bring tons of media attention, and honestly, it would be kind of hilarious.
Buuut, there ARE those rumors about a Paper Mario game potentially happening and all that jazz... which makes me wonder if they could instead give us a mario RPG character. I don't care about Geno, but that could be a possibility. My preference would be paper mario. Or heck, just go nuts and wild and give me paper waluigi. Or Rawk Hawk. Honestly, I don't quite care, I think it really will just be waluigi.
Honorable mentions:
Zelda, Kirby, DK - Depends on how shill Nintendo wants to go. I can see a new Kirby game happening soon, BotW2 is happening, and it would be cool to be in a reality where Dixie gets some love. If anything, there's a possibility they skip the pokemon DLC rep and focus on a different property.
Kid Icarus, Mother rep - extremely less-likely, but I want to believe there's a chance these get some attention. Uprising deserves a re-release on Switch, and one can always dream of Mother 3.
Honestly, all things considered there's a ton of really well-known characters that could be added from third parties that I don't even want to go there. Otherwise this will never end.
BONUS ROUND!
Mii costumes
- Cadence (Crypt of the Necrodancer - Swordfighter) w/ Disco Descent (song)
- Knight (Hollow Knight - Swordfighter) w/ uuuuh... I have no idea which song is the best from here, tbh. Honestly, I would prefer Hornet too...
- Madeline (Celeste - ???) w/ First Step (song) ... I don't think Madeline from Celeste is exactly a fighter, mostly just want some representation and music from it.