I actually had high hopes for a James Franco-Multiple Man movie.
Civil War made the line between solos and teamups very flexible. So concievably anyone could hold a solo if you pitch it right.
I imagine Disney+ shows are gonna replace solos in the MCU.
10 years ago no one would have thought a Carol Danvers movie would have made over $425 Million at the domestic box office or over a billion globally either. So, it's not necessarily the character's appeal that can drive a movie like that. The interest in Avengers and how Captain Marvel would impact Endgame was one of the reasons for that movie's success.
The interest in the X-Men relaunch and how mutants would be integrated into the MCU could work in the favor of whatever the first MCU X-Men movie. It could be Maggott the Movie and people would go see it.
Rather than starting with a big X-Men movie with all the characters, starting small with a single character discovering he/she is a mutant and then discovering the wider mutant community is one approach they can take. Kitty has been used as that type of doorway character before.
Ant man succeeds by borrowing from heist films. Aquaman from fantasy adventures.
Who among the X-Men have that crap to pull from? Wolverine succeeds as a western, Deadpool as irreverent action comedy.
Magik, fantasy adventure. Gambit, heists. Bishop, buddy cops?
Just throwing it out there.
Strictly solo? Probably none of them. Small (2-3) ensembles? Any of them. Captain Marvel and Ant-Man are not as much solo movies as small ensembles.
I could definitely see a Gambit-Rogue-Storm movie. Kitty-Wolverine-Colossus. Nightcrawler-Wolverine. Psylocke-Archangel-Iceman. There are endless possibilities depending on the story you want to tell. Cyclops-Havok-Starjammers. I could probably keep going.