According to the game, Bismarck's element is Wind and Water
Ravana's element is Fire and Earth

it seems to suggest that the next one will have the dual element of Thunder and Ice, but I can't think of any existing summon from previous FFs that may match that exact description.

Tritoch/Valigarmanda does come close because its very first appearance was it being trapped in ice and shocking the magitek riders. But it's more than just that because it's also commands fire, thus, making it a triple element summon.

Then again, Bismark, afaik, was purely water element in previous games. It's possible they could just re-imagine an old summon and add one more element to it. Or it could be a new one.

What do you think.