Well after having the first DLC with Freyr as 4th Norse Mayor God, here it is my suggestion for the remaining Pantheons to have in count for the future.
Atlanteans: TYPHON
It could add a very aggressive style with new creatures and destructive godpowers.
Greeks: DÉMETER
She was often considered one of the 12 Olympians and it could be a nice addition giving an economy boost, especially for farms. Her powers could be focused on the Economy.
Egyptians: AMMYT
She can be the Egyptian version of Freyr in some terms. Defensive powers in early, but destructive ones in late game