A puzzle battle against nature. Grow forests, care for trees, keep nature balanced … whilst fighting off the enemies ( = which is just a metaphor for humans)
Levels? A nice nature-y area. (Orthogonal side view in 2D? Or make it actually 3D?)
Input method? Something that resembles “growing” and “trees”. Maybe you can only give water to a square? Or only plant seeds?
Or there are a few default actions you can take, which all resemble controlling the elements. So you can provide water, sunlight, a wind, etc.
Goal? Win the battle. (Wipe out the others? Or create a balance? Just survive for X turns? What is it?)