For this reason I feel, if anything, SEGA is worth a lot more discussion than it's getting right now - I think if any of the current major players get something, they are far and wide the frontrunner. No content at all in this pass, making it the only totally absent third party other than SNK... which is a bit more debatable. I've been trying to pioneer that a bit but it's hard, because at the same time it's not like we have a ton of options that feel plausible either.
But let's break it down a lil for the sake of discussion, since I've already got those gears turning.
Eggman is the only one that sees consistent discussion, which is funny because I'm entirely convinced that Eggman is not happening due to the particular way Sephiroth was promoted. I'm more partial toward Tails' chances, but it's hard for some people to digest that idea with his Mii Costume existing (even within a grey area). In general I think more Sonic is the most digestible for most, and it makes sense since Sonic is by and large Sega's most profitable franchise.
Outside of Sonic there's Arle, who I will always campaign for but I've lost a good deal of confidence in... but Puyo Puyo is a massively significant series, Sega's second biggest and one that has had longstanding history and presence on Nintendo. It's hard for a more abstract character like her to take off because of her cutesy appearance and unconventional genre, although it seems like most people at least acknowledge her as a possibility.
Kiryu is in a weird position because of contradicting interviews, but neither of them really help him out very much. From an interview saying they don't believe Sakurai wants Kiryu in Smash, to the other where they say it would be wrong to have Kiryu punch women. Even if we take the latter out of the picture, the former isn't great. Otherwise Yakuza has really been growing in popularity and would be worth keeping an eye on.
Sakura exists, and Sakura Wars has been emerging as a major player for Sega and inching toward more of a western presence... but I think she needs a little more time. This is one of those characters where I could see them having a much stronger case next time around. As of now I think Sakura Wars just has some more growing to do in reaching a wider international audience.
And then there's still an opportunity for Shin Megami Tensei, most notably Demi-Fiend I would say. It's been a bit unfairly counted out because of Joker, but Persona really is treated like its own separate thing and explicitly SMT content literally doesn't exist in Smash yet. SMTV is on the horizon, Nocturne remake is a thing, there's at least a case to me made for this one but there's always that slight dissociation because Joker made it.