Hey, I've found your mod while searching for the mods which will add some sort of harmful spreading creep mechanisms instead of biters. And your mod with trees spread is looking like an awesome starting point!

I have a list of ideas and maybe you'll consider some of them interesing for you to implement:

• trees more resistant to damage and harmful to all the buildings near them, not only the pavement
• trees auto-targetable by the turrets and concidered as an "enemy"
• tree mutations aimed to protect the trees from the harm they are experiencing, similar to the evolution levels of the biters. Leading to increasing threat to the buildings and higher resistances to the damage.
• trees somewhat able to pass the water by slowly turning it into wet swamp.
• biter corpses spawn trees with some chance
• trees spreading something like non-targetable spore clouds which will, with little chance, spawn new trees randomly along the path of the cloud, even inside the base
• biters reacting to the trees being attacked if the forest is connected to the biters base
• trees slowing the player by noticeable amount while speeding up the biters

Very interesting ideas, I've had some of these brewing myself already but likely in a different mod potentially called "Revenge of Gaia" but my life has been real busy and still haven't managed to find the time to play factorio space age.

I might mess with something like this if time permits in the future.