If billboards will be implemented in the future, it would be perfect if they work in the following way:
1. use billboards from a certain distance to the camera
2. color, size and image variations
3. face to camera (that has been mentioned here already I know)
4. pre-rendered animations for movements and other changes (spring to autumn) in the distant trees
5. is it useful to have images from different angles for the trees? (birds-view, front-view ect) or is that not necessary for distant trees?