yes, I thought of outdoor levels, as I work only with them.

I'm also making some terrain samples for Heelx new terrain shader, so I can make for you a few ones too, just tell me an approximate size you need, like 1000x1000m or 20x20km (you can tweak the vertex resolution in MEDD), and also can tell me some other parameters like how flat or mountainous it should be, lakes, or sea, or rivers are ratio etc. I probably will generate it in L3DT and import to MED. file size mainly depends on textures, I can use 512x512 tileable ones, they should be fine.

earlier I made a set of vegetation models for my editor, you can use them freely, just mention their source in credits or the readme file. and I have also some other free stuff like building which are free to distribute.

what could be important in your case is file format, as wmb does not allows to compile several thousands of trees, but you can use my file loader easily. it creates entities simply by ent_create based on data stored in files.