The image shows the main path through the game's various zones. At the moment it just represents the shortest path from A to B but will eventually be weaved into a more complex network of intricate tunnels and caverns.
On previous projects I've often spent a lot of time recreating detailed level design sketches in the editor, only to find that the scale is way off throughout the whole level. One of the things I love about Unity is how easy it is to quickly chuck in a load of prefab shapes and copy and paste until I have a simple layout ready. Then with one click I'm swimming around in the game engine and very quickly getting an idea of whether it feels the right size or not.
The world of Luminesca is still in its early stages but it feels like a good size; it's big enough to give a sense of grandeur without being so big that it's unreasonable to try and fill it all with enough detail.
The next stage is to look at each zone individually and start implementing the main gameplay and story beats. I'll start with the mid-game section before book-ending it more simple and more complex versions of the gameplay.