In the comics Galactus simply isn’t a villain and I don’t want them to make him one like they did with Ronan. Kang would be cool as there lots they could do with him that would be very unique.
They could also have Annihalus and his Wave; bring back Ultron and have him become a cosmic level threat; or maybe do something different and have Norman Osborn being both the Green Goblin and the leader of the Dark Avengers with his own Iron Man armour. He would be more of a political opponent would be a dark reflection of Fury, doing little cameos and recruiting the Dark Avengers (Abomination could be a member).