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.
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.