Honestly? I'll make another case for Chun-Li here. Street Fighter 6 has been a rousing success and helped revitalize excitement for the series in a fresh new way. Surely Street Fighter was doing fine enough to warrant Ken in Ultimate, but I think there's something to be said about the way Smash treats Street Fighter vs what the series has built up for itself recently. In Smash, the series remains a celebration of the classical Street Fighter II era - something I've rambled about before. But now I think SF6 has brought attention back to everything beyond that, and would demand more current focus.
Another thing to add is that Chun-Li is one of the more recognisable female characters within the fighting game genre.
Other than that the Street Fighter franchise does have a lot of interesting and iconic characters too whether this is from the likes of Guile, Cammy, Juri etc.
On the highlighted whilst Street Fighter 6 certainly helped revitalised Capcom's fighting game department (especially with the Marvel Vs Capcom Fighting Collection: Arcade Classics), I do hope that they will consider revitalising several of their dormant fighting game IPs down the line.
On the topic of fighting games, definitely Sol Badguy or any Guilty Gear character. The release of Guity Gear Strive introduced the series to a bigger audience than ever, firmly taking the series from more of a hardcore, cult classic status more akin to Darkstalkers into sharing space with the biggest names of the genre. It seems safe for me to say that if we get another fighting game character next game, it'll either be from Guilty Gear or maybe Virtua Fighter.
I generally could see Sol Badguy or any other Guilty Gear character being represented in Smash for sure. As for Virtua Fighter, all that is known so far is that a new entry is currently in the works and I think they may possibly reveal it some point next year.
I would also perhaps throw in another possibility of another fighting game character represented from the SNK franchise in which they are starting to gain recognition. My predictions would be between either Rock Howard, Mai Shiranui or Nakoruru.