@Nightshade , did you also check out the Wood and cave first seed DLC? I know it isn't a crypt or castle, but there are some wonderful cave variations ( coastal/underwater is neat one) There are a lot of ruins covered in grass/moss mixed in with the forest tileset stuff as well.
If we are also suggesting possible future DLCs in this style then I would love to see Desert and Volcano/Wastelands variety!
I just successfully tested changing music by region, and changing it between continents on the world map after getting on/off the airship. I had to override the default audio transitions with eventing to make it change smoothly but it worked.