Hmmm yea I definitely think that's possible. Usually the event announcement hints at an element through description or boss pictures but those don't seem to be present.
I really like Cleo and Elisanne but I hope they aren't on the banner or at least 4*; more characters should have a chance at alternates first. Maybe Orion, Ezelith, and Hildegarde as banner units and Cleo/Elisanne are just the story skins to promote the event? It's hard to tell, but we should get more info soon!