Here's a quick block out on just the thought process I have for this.
I was just adding a bunch of boxes around on what I was thinking it would look like, just from the first image, and then built around it (which probably isn't as necessary). I plan on re-using assets that I would make to fill in the areas around the main spot. Just for the sake of paths and what leads to where, I've added point lights on areas where it'll almost serve as an exit or entrance to somewhere else (these will eventually be replaced).
As far as what I'll be making next, I most likely will give the ground and water a first pass, with some temporary meshes along the way, like some rock-type meshes and a bridge mesh.