There were High possibility of the Galactus or Doctor Doom as main villain in future movies.

There were rumours of galactus coming out in Endgame but obviously we didn’t see him, but Superskrull would be interesting to see or Dr Doom, this character has already played a big villain in some animated series’s.

Maybe Kang the Conquerer?

In future MCU movies we could expect cosmic entities as the villains or some others like kung the conqueror or collector

Glactus can be appearing in future movies of MC movie