Fun game! 100% needs a lightning upgrade, because all incremental games need a lightning upgrade!

But upgrades that leave damage/status-effects on the ground such as burning, poison, another freeze, lightning clouds ⚡ ☁; those would all be cool in future updates.

Enimies that buff each other would also be good, such as healing or reducing damage in an AOE around them so you have to priotize certain types of enemies would also be good.