My pick is Dante - not sure if that's bias or optimism on my part, but I do have reasons for it.
I'm guessing it's going to be an adult/teen human character, since DLC tends to lean that way. (not counting veterans, there's
- I assume it's because it's easier to use other characters as a base) There aren't any recent/upcoming Nintendo games with humanoid protagonists that could be promoted through that spot (there's Metroid Dread, but I'm not sure if they'd add the EMMI robots as full-fledged characters), and while they could announce something new (like a Xenoblade Chronicles X remake) and put in a character from that (they did it with a Japan Direct that showed a Xenoblade Chronicles 1 remake for the 3DS and followed it up with Shulk in Smash), I have my doubts that they'd do that.
I made
a list of characters that I thought were plausible earlier, and out of those, the adult human ones were Elma, Sol Badguy, Ryu Hayabusa, Dante, Phoenix Wright, Morrigan Aensland, and maybe Ezio. (I have my doubts that they'll double-dip on third party companies that are already in this Fighters Pass, so I don't think we'll be finishing up with a Namco-Bandai, Microsoft, or Square-Enix character)
-I'm not sure if they'd have two Xenoblade characters so close together; I don't know if they'd want to risk making it feel over-exposed like Fire Emblem
-Ryu Hayabusa doesn't seem likely, given how
a bunch of interviews surrounding the Ninja Gaiden collection for Switch have gone.
-Phoenix Wright would require them to make up a completely new playstyle for the character; his home series has no combat (since it's a game about lawyers, it's largely debate) and his Ultimate Marvel vs. Capcom 3 playstyle doesn't seem like it would work well in Smash. (it relies on gathering randomly-generated evidence until you get enough good evidence to enter a super mode - it doesn't seem like it would be fun to play against in Smash's larger stages, since it would turn into keep-away, and it doesn't seem like it would be fun to play, since it relies entirely on randomness for him to be useful) I could see the development team appreciating the challenge, but given how tight the timing seems to be on DLC characters, I'm not sure if they'd include him
-I'm not super familiar with Ezio; maybe they could include him, but I'm not sure how well he'd represent Ubisoft as a company
-while I could see Sakurai/the developers wanting to be the ones responsible for bringing Darkstalkers back into the public eye and possibly raising interest for a revival, and lovingly crafting a stage, spirits, etc. to pay tribute to the series' history, I have my doubts that Capcom would agree to including Morrigan over a series that they're more likely to profit off of, such as Monster Hunter, Phoenix Wright, Resident Evil, or Devil May Cry
-I could potentially see this going either way between Dante and Sol Badguy - on one hand, maybe the developers would want to cover a wider variety of genres, but on the other hand, they've had no problems with including several RPG characters back to back, so maybe they would have no issue with including two fighting game characters back-to-back
I don't see the same sort of issues with Dante - they're already working with the company that makes his games, it's an active franchise that I could see them wanting to promote, Capcom's the only returning third party company without a unique newcomer in Ultimate (though maybe they're fine with Ken and the two veterans), and his moveset writes itself. My main concern is that a bunch of DLC characters already have elements of his gameplay in their moveset (
= Rain Storm,
= Round Trip, possibly
with Devil Trigger, arguably
with Weapon Switching), but I'm not sure if that's a plus, a minus, or something neutral. (since Dante has more than enough material to work with even without those)