I've just been thinking about how generated structures, and more importantly the landscape, will be added, when it comes to that. I got to the falls of Rauros and realised that this most important of water features in Middle-earth may not be added due to the way that rivers are generated. It would mean that the whole of the Anduin north of this point would have to be much higher than any other river. In all, it would be tricky.

Does anyone have any thoughts on this, or any other difficulties with large structures?