I doubt it is Kang because he will be the next villian in the ant man series, plus Jonathan Majors isn't a big name and doesn't yet have the acting chops of a Josh Borlin.
Doom is a possibility but he isn't the same level of threat as Thanos, he is basically the evil version of Iron man mixed with doctor strange. However he would be able to pull off the same anti hero vibes as Thanos with the right actor.
Galactus is an existential threat as he is a planet eater, that could be interesting but he is a huge ass unknowable entity so it wouldn't be as much personality and characterization as Thanos.
Doom is a possibility but he isn't the same level of threat as Thanos, he is basically the evil version of Iron man mixed with doctor strange. However he would be able to pull off the same anti hero vibes as Thanos with the right actor.
Galactus is an existential threat as he is a planet eater, that could be interesting but he is a huge ass unknowable entity so it wouldn't be as much personality and characterization as Thanos.