Sorry to bring you down @Wetter42, but I'm afraid you won't be able to start your modding career with something spectacular right off. It takes a lot of learning time to get where some of the most prolific mod authors currently are. Now, I myself am not an expert, but I can tell right away that map modding (other than scenography / content creation) is not a beginner's thing...
Actually, some fail even at content creation...
You might want to talk to people like ryanm2711, jin007 or speedium as they have been creating new lands within the game world from scratch and, probably, according to the workflow that you have described.