Why don't you start with something simple? with a hangout map, for example:
A Cold Valley(winter), with open room to run around freely, with frozen grass/snow, some half frozen ponds with a few holes where you can dive into, and some mountains, rocks, a cave on a wall (so you dont gotta use too many FOFs), and any other natural things.
Edit: Whenever you got a prototype for a future map ready, feel free to share it here to get some feedback to help you finish it, so you can later publish it as a regular addon once its completed and polished.