I once played an eladrin wildfire druid with a homebrew rule: as her season changed, the damage of her wildfire abilities changed to match: so in summer mode she was a wildfire druid, in winter mode she was a wildfrost druid, spring was lightning and autumn was acid. There's a loose elemental association here (spring = air, summer = fire, autumn = earth, and winter = cold).
This applied to produce flame, any spell granted by the subclass, and subclass features (like the companion's attack).
I was never really satisfied with autumn being acid, but I've never come up with an answer that sat particularly well instead. I suppose spring = radiant and autumn = necrotic could work, but that has potential balance issues. I never really found a good answer for "earth damage" in DnD.