Looks like very polish and clear!
I have a technical question about how you are implementing UI, are you using Slate in C++ or Widget Blueprint (or both) ?
Another question about modding, how you will manage it (if you will allow it for UI)? We will be able to change UI pieces (or entire UI) only via UE4 Editor or you will allow users to edit pieces with Photoshop/Gimp without need to use Editor?
For example, i'm working on a small music player blueprint actor for the UE community and for the UI i read the .png files for the brushes directly from disk at runtime, obviously in this way the user not have a complete control over the UI like move around buttons etc. but he can mod the skin colors and icons "easily" like this