How about making each season’s colored gem ‘dominant’ for that season? Example: Space Season is blue.
A few options:
-That season’s dominant gem drops more in gameplay
-That season’s dominant gem powers do more damage or have other increased effects
-That season’s covers that are dominant gem powered drop more and/or cost less HP/CP to purchase or power up
This would shake up the meta a bit and we could see some fresh team combinations.