Who do you think will be the villian of GvK 2

Gigan could potentially be a good choice. He's the most unique out of the three and his origin could work as a "ancient undiscovered threat". He could be another alien titan that's currently lying dormant in a mummified state (similar to his appearance in Final Wars). Being an alien, he could also be tied to King Ghidorah's origin as well. 

Though, more than likely it'll probably be a new monster.

I'm still pretty adamant about Biollante, it'd be a cool idea for an "ancient threat".

SasquaDash Agreed, Gigan would be more likely to appear again. But Biollante could be interesting, it would be like the fan movie Godzilla: Desolation but with Kong and other different outcomes and stuff.

I know it's gonna be Destroyah because legendary said it will be a "massive" threat