I said I would dwell on it for a little longer, but I’m tired. Spike (the Spike?) wrote this awesome thing where you can add heightmap based terrain to a map, just like in Crysis. It’s an in-engine editor. You can sculpt and paint the terrain, you can make holes through the terrain and build brushwork through them, you can place models on the terrain. There is also texture blending.
The terrain sections are somehow ingeniously cobbled together to load only what’s needed, so the maps can be large. Like 100×100 miles or something. Large. Collision seems to work perfectly, I haven’t really gotten stuck or fallen through anywhere yet.
So, er, let there be rock? Like, massive massives of rock.