Adding new locations, then a totally new world

My idea is to make a lot of alternative overmap tiles, test them on the normal world and when I have made enough use them to build a new world quite different from that stale New England setting.

First of all, how can I make a new overmap tile? I can put a “mapgen” folder in my mod, make a .json with the map of the new location, but I do not know how to integrate it in the newly generated map and how to test it. AFAIK I should make a new overmap_terrain.json in my mod folder, add the data for my tile, then generate a new world. Right? So, what’s the difference between overmap_terrain and overmap_specials?