I've been adding in modding support and wanted to share its progress.
The modding feature allows a player to direct the 3D procedural generation (biome layout, enemies, objectives etc) by way of a simple 2D map editor.
The map editor enables hassle free, quick and easy creation of worlds and challenges by allowing the user to simply paint layers onto the map (a grid). Once saved, clicking play launches the player into their creation.
Painting a city and enemy layout and then flying through trying to survive your creation is actually a lot of fun (see below for a work in progress preview).