Just making the campaign map a bit more modding friendly would be a start. It's an absolute pain at the moment just to tweak a bit of coastline a bit, far less to make any more drastic changes; it takes forever, and it's almost impossible to avoid making errors which break the map. An actual decent editor would be a godsend; as Ibn-Khaldun points out there is already a battle editor, so I don't see why a campaign map editor would be a huge extra burden.