It might be cool to make all these UI elements resizable and movable by client-side mods.
You could also let server-side mods add UI elements for whatever gamemode the server is doing
That should be very easy to do. I'm not sure how that would work exactly just yet, but it could be something as easy as:
Image window = GameObject.Find("Player Inventory").GetComponent<Image>();
window.scale = new Vector3(0.5f, 0.5f, 0.5f);
window.position = new Vector3(-100f, 50f, 0f);
That's a good idea as well. It might be best to let players download mods from a host (similar to RTB), in order to have to use required GUIs for the servers. Although, there could also be builtin client-sided functions for adding GUI elements (or sending limited data to them from the server), which can be called by the server itself for specific clients.
Sorry if this has already been asked, is there going to be VR support?
Not sure yet, but we'll definitely look into it. I do have a VR headset, and Longor just bought one himself.