Kang lost his viability as a villan when he was beat by ants and Antman. How can he suddenly become a that to the Avengers in any form? Galactus, while a great villian is not something that the audience can see being punched in the face by Hulk. His scope is too big. Maybe if they tease him with a few of his champions? They need a villan that is touchable. Annihilus maybe? Mr Sinister and his acolytes would be good to usher in the mutants.