How expansive can modding and mapping be? can players, for example change the art style?
About the only thing you can't mod easily is the GUI - which is all unity prefabs, I've compensated for that where possible (by creating dynamic resizing and basing everything off of grids so that you can create new types) but it would still be a pain to, for instance, create a new window or completely change the layout of one.
If you watch the most recent dev video you'll see how the terrain is painted - it just takes a 200x200 texture and pastes it over an alpha mask. By swapping those images up you can create a completely different map style (like seeing the Cartographic vs Realistic style in the map) - swap out the mountains/forests/coasts and you'll have en entirely different map. Of course, if you change the aesthetic too much (make it very dark for instance) you may then have to modify the POIs to stand out better from the map - and further artistic work may be needed to complete the overhaul in a satisfactory way.
You'll be able to find all of the raw pngs in the folder structure, and duplicate to create overrides or alternate map packs. We package up the separate sprites at runtime into atlases for performance reasons but defer the process to make modding as easy as possible.
As far as modding outside of the map - you can create a global mod, scenario mods, and specific rulesets that can be applied to all scenarios. Everything is built to roll together seamlessly. You can use the in-game engine, code custom callouts in a .NET 3.5 framework, or modify the JSON files directly.