This post gives inspiring ideas.
I've no experience about map making (for now) and i'm so busy with my mod, but i found a lot of inspiring tree assets (with low poly counts) that can be used for making very cool forest maps.
All the map i tested got nearly all trees have the same height. I didn't see any huge trees (like in avatar movie)...
These are low poly trees (there are some blender lowering polycount scripts for higher count models)
https://free3d.com/user/kim/models/4
https://free3d.com/user/kim/models/5
..
Today, 10 years after supcom FA release, we have access to huge amount of ressources to make fantasic work on supcom/map modding.
If we add a sim script to theses maps, we can add some aoe properties to some trees (healing trees, shield regen trees)...