I know that this has been suggested before, but I'm really needing the built in Unity terrain engine to accept custom geometry. Currently the only way that I can place trees with culling/billboarding requires me to hand paint a unity terrain under my custom geometry. needless to say, this is VERY time consuming, especially when trying to place grass/rocks at a ground level. There are several work-around solutions which have appeared in the asset store, which allow you to place objects on custom geometry, none of them utilize a billboard culling system like the built in Unity terrain solution does. This is must for optimization playback. The Unity terrain does everything else so well, it seems odd that this seemingly simple addition has been left out.