I totally agree with the idea behind this thread! Not a foliage system for trees though (these should be placed as misc_gamemodel IMO), but for grass and small plants. If possible, done via shader file (specifying a 3D model that can be randomly scattered around a texture). Would be very good and useful to have such a system.
This ability however might already exist in Xonotic! If anyone still has the old Nexuiz, take a look at the map ONS-Reborn. It uses a shader to randomize grass models across the ground. The grass there is a camera facing sprite, but I expect the same could be used with 3D models. Worth checking IMO, so the same thing isn't implemented twice.
As for foliage models, I agree we need more trees and grass / small plants. I'm barely a modeller though, and couldn't succeed much at that.