The underground system looks awesome. Can you make some of a water feature in there?
Possibly.
I'm debating right now whether or not to scrap the whole "exterior" portion of the map, that is, the expansive hilly area outside the city walls (the walls that will be there when I'm done) with lots of open area and a few scattered houses/garages. I initially liked the idea a lot, but I feel like if I don't get the implementation perfect it will make the map look odd or just be somewhere no one ever goes.
The plus side of taking that out is that it frees up a lot of my geometry budget and I can do more buildings in the central area and possibly a more robust underground.
I like and dislike both routes, ultimately I think it will depend on where I am budget wise when I finish this central area.