Neo Tokyo, Elin Forest, and Tonosama

The possibility of GMS getting Kaede Castle (The fuedal area) is very low.
And if they did release it, it'd be changed like Shouwa because it features human enemies with realistic weapons (swords, knives, and throwing stars).

gMS got Showa because another version had already edited a "harmless" version beforehand, and it was easy to just copy the data over. So unless another version edits the "dangerous" stuff from Kaede Castle, I don't think Nexon would go to that much trouble to edit all the mobs and stuff.