Step 2: tracing in Illustrator done!
I've added lakes, rivers, minor rivers.. and mountains.. yep, I borrowed mountain vectors from here. This project is intended for personal non-commercial use and I didn't plan putting too much work in it, so I found this resource essential.

Forest were made using "symbols" along with the "spray" tool, a method using a reference/instance system, letting you place same instances in an organic way (as a spray) and modify their single reference at will.
Anyway I hope to find a better way to render trees later in Photoshop, I need them now to figure out where important points of interest are in my setting.