I would prefer them to stick with the trend of representing series that have yet to have a character in Smash, but I don't have any specific preferences at this point, either way. I feel first party picks might be less biased since Nintendo representing Nintendo in a Nintendo game won't offend third party developers for not being picked, whereas when one third party developer gets a character and others don't, that might be a little upsetting for those who got overlooked? I like seeing third party characters, though, because it really makes the competition between companies seem friendlier than it has been in the past, which is pretty refreshing. I could see either first or third party reps happening, and I sort of anticipate a mix of both from here on out.
As for what's likely for third party, there was a tweet made by Hideki Kamiya near the end of last year--that has since been removed--that had him hinting at another one of his characters being in Smash, but I have no idea if it was a hypothetical tease or not. But, meme status aside, I feel the likelihood that Dante could end up in Smash as DLC is relatively high (????), given the timing of the Switch port of Devil May Cry. Amaterasu was also hinted at in his tweet, which Okami got an HD remaster on Switch in late 2017. I don't know Capcom's exact current relationship with Nintendo, but they do have a character in Smash already, so it seems workable. Only thing is I feel like Dante wouldn't really be much of a curveball because I think he's in a lot of people's predictions for a wide assortment of reasons. Amaterasu would be interesting, though.
Onto first party... even though I'm not that picky, as thePlinko said, I would
love if we got another Zelda rep; doesn't matter who. If they do end up going for characters from series already represented (which will probably happen if they go first party again), we
need a new Zelda character. I'm not resentful of all the swordfighters in the game, but I
am annoyed that Fire Emblem has more reps than one of Nintendo's most successful franchises. Fire Emblem has different character casts each game (yeahhhhh a Three House rep is another to add to the "likely" pile), which I get is different than Zelda or Mario where at least the main characters are consistent across the games--so that's a big reason to why Fire Emblem's got so many characters in Smash. But, yeah, most of those except like Marth and Ike, are one-time characters. There are a handful of one-time Zelda characters, who could be extremely workable as a playable fighter with potentially unique movesets. I don't know who'd be the most likely, though, because all the best choices (Midna, Ghirahim, Skull Kid,
The Moon) are already assist trophies. Hyrule Warriors already has a lot of characters given movesets for ideas that could be useful in envisioning something, but I don't actually know how probable another Zelda rep is, even if I do want one. If I take a crazy leap, I could fall back on my Capcom conspiracy theories, and suggest they throw someone like Vaati into the mix.
Like I said above, Three Houses will likely have a rep at some point. Byleth is probably guaranteed to be that character since we really only ever have gotten the main characters from Fire Emblem titles in Smash. He might be more different as a sword fighter, probably will have some similarity to Simon--but I'm not sure if I really like him as the "best choice" for the Three Houses quota. Even though I am okay with the abundance of sword characters in Smash--I say this a lot--I'm bummed that it's always swords, and never another type of weapon. As much as I dislike Edelgard as a character in Three Houses, I'd prefer her for Smash over Byleth because she'd be the first Fire Emblem character to not use a ****ing sword. I'd say Dimitri because I love spears, but Edelgard is overall more popular, so I'd bite the bullet for Edel just for someone to be in it with a different type of melee weapon.
Been said a lot in here, but a gen 8 pokemon is pretty probable as well, but I'd still prefer Sceptile as that final grass starter evolution.
I'd been thinking about this a bit recently, and I kinda want to mention it here as well; the concern with a lot of characters that people expect/want, is how similar their potential moveset is going to end up to other existing characters, for one reason or another. This feels especially true when the discussion of swordfighters comes up. When the roster is pushing, like, what--80 characters now?--it's difficult to design an attack that doesn't at least somewhat reflect another existing character. I am actually am okay with this--different characters with attacks that function similarly to another allows some nitpicking on the player's behalf when it comes to finding a character with the perfect moveset for them. You end up with a lot of characters that are later additions who are an amalgamation of previous fighters, and it's not necessarily a bad thing. If you like the way Sheik plays but there are just certain things about her that do not work for you and you cannot fully utilize, you have characters like ZSS and Joker who feel similar to play for widely different reasons that might be a better choice for you. I play Smash on a lot of muscle memory, so my mains incidentally have very similar attacks--especially when it comes to their aerials--despite being really different characters that probably otherwise wouldn't, or shouldn't, be compared. So I'll defend the lack of creativity regarding new characters' movesets for that reason. The special moves is what really makes each fighter unique--or, ha, special.
And finally, my "fever dream" character, as I like to call him, is Raguna from Rune Factory, but you'll never hear me actually being serious about voicing my desire for him to be in Smash nor would I ever expect him to be in Smash. And the former is entirely is due to the latter. But a man can dream.