Hello dear Epic,
I would like to post a short list of features, that in my view, would greatly facilitate working with landscape system, especially for large open world games.
1) Add an ability to paint color map on terrain in the editor.
2) Add a feature, similar to landscape grass, but for physical surface id, so that you could control distribution of landscape physical surface types in a shader.
3) Add an option to change shaders on individual landscape components depending on the distance from camera to the component.
4) Somehow expand functionality of custom material node to support outputting several values. For example, returning float4x4 gets broken down to 4 pins in the editor. This would probably be best material editor change since release.
Last feature is not directly related to landscape, but it would prove to be really useful.