This like what I've seen on some of the videogames I play: the trees at the foreground are real 3D, and the trees further back are flat 2D images. Since the player is not supposed to get close to the 2D trees, they won't notice they're 2D and low-detail. But I suppose you want to have a more free camera, and if I remember correctly, Ancient Savo uses algorithmically-generated maps, like UnReal World?