I've been playing around with the map editor, and one thing I've noticed that's a little annoying is that you can't create forks in rivers. Also, you can't really create a river with a body of water as its source. You kind of can, but it doesn't join the little pool to the body of water. Both of these would be nice from a map-designing standpoint, and they don't seem like they'd be too hard to implement, though I'm not entirely sure what rivers actually do, so I'm not sure about that.