Kroiga
Raban
A new, bigger Anguirus
Dagahra
Mechagodzilla (Post credits scene, duh)
Megalon (Maybe his origin can be that he was a mutated member of the Hybrid Kumongas)
Hedorah (Maybe the blue Hedorah/Yog like things inside the Hybrid Kumonga that got mutated into Megalon came out of him and mutated into Hedorah)
Behemoth (Toho needs to use him, as long as they get the rights from Warner Bros. and Legendary)
Biollante