The Fantastic Four reboot continues to be quietly developed under Kevin Feige but according to sources, Marvel is currently deciding between Kang and Annihilus for who will be the main villain.
Kang a time traveling conqueror with vast powers and technological supremacy. He has taken every opportunity to torment the Marvel Universe and is one of the Avengers' greatest foes. While Annihilus is a a native of the planet Arthos, is the ruler of the Negative Zone and a longtime foe of the Fantastic Four. He is a nihilist who is obsessed with constantly finding different ways to extend his lifespan and isn't averse to destroying all that which he sees as a threat to his existence.
Kang and Annihilus is similar to Thanos, they have the potential to be a big threat to the MCU. Both Kang and Annihilus have almost succeeded in defeating the Fantastic Four and the entire Marvel heroes.
Kang as a major villain for Fantatic Four reboot would be a perfect choice, now that the MCU is open for time travel like the Avengers: Endgame. While Annihilus a major villain also make sense, with references to a city within the Quantum Realm and its connection to the Four already having found mention in the movies.