Add Mutated aphids the size of basketballs. They would have an armor piercing proboscis, adapted to penetrate the bark of trees and feed on the sap. Passive unless angered, and can be domesticated to allow players to harvest honeydew (a number of ant species actually do this), which can be processed into sugar or other useful products.
@kevin.granade @Trigon Make the Giant Ants have evolved immunity to the Fungus, and utilize it in a form of simple agriculture, much like IRL leafcutter ants. Except in this case, its not mutualism; the fungus DOES NOT want to be eaten, and isn’t being given a choice in the matter. Giant Ants feeding on the fungus would also help check its growth, which would outright destroy the ecosystem if left uncontrolled. Ant agriculture might also preven the ants from outright stripping the entire environment of biomatter, since they would be growing a lot of their own food. Though they might supplement their diet with zombies or wildlife given the opportunity.
Also, regarding having more plant life, this thread I started a while back. The thread kind of shriveled up and died pretty quickly, but it had some good stuff I think. Mutated/Invasive Alien Flora?