One thing that would really accelerate this is better use of transparent alpha channels in OpenGL. I'm a big fan of applying masked images of landscaping on a couple of transparent walls or objects and intersecting them in an "X". In Lightworks, this works great, especially since there's a wide variety of images available (see http://www.imagecels.com). No library will have every tree you want, but you can get or make images. They even make decent shadows!
However, OpenGL makes the frame black, not transparent. I like using OpenGL to show clients real-time flythroughs, but I can't use these trees and plants, as they're too distracting. Modelled trees are not a good sulution. They're rendering hogs that never quite look right. Tweaking OpenGL would be the solution.
While we're at it, can we add shadows and hidden line to OpenGL? The we can get rid of the "internal engine" from the 90's altogether!