The map mirroring tool I made for Shadaoe wasn't released for two reasons:
1. It was hacked together, not user friendly, and only worked on the terrain tiles Shadaoe used on the map because I had to write rules for how each terrain type would be rotated after mirroring (if you don't rotate them the tile transitions don't line up at all). It was also unable to mirror the tile transition data for the mirrored side of the map, meaning that you couldn't properly edit the terrain on the right side in Krom's editor. All of these things could be fixed but would take a lot more time than the hack I made.
2. I didn't want to see every map maker creating "mirror maps". The Valley of Dangers is a special case, each player is supposed to be perfectly equal. For example, these maps could be made by mirroring: "Valley of the Equilibrium", "Dead of Winter", "Enter the Heat", "Cross", "All for One", "Horseshoe", "Alice", "Multilands", etc. However, because each side was made separately, they're much better quality and more interesting. For example Dead of Winter looks quite similar on both sides, but if you zoom out and look at both at once, you'll see that the river and bridges are actually in quite different positions and the surrounding terrain is different. If people did start making lots of mirror maps we might not add them because in a lot of cases it's a "lazy option" and results in a far less interesting map than if you make both sides slightly differently. Perfectly mirrored maps look quite unnatural, Shadoae spent quite a while swapping objects in the middle valley so it didn't look so mirrored. But some people might have their own ideas like The Valley of Dangers that could be made with such a tool.
I still have the code and maybe one day it could be implemented into the editor (copy -> paste mirrored). But it would be released with a disclaimer: "Mirror maps will only be added into the Remake where the mirroring is used in a way that makes the map better, not just to save some time"

(I believe The Valley of Dangers 2 fulfils this perfectly)
Does anybody have an opinion on the matter? Do you think such a tool could be used productively and without too much mirror map spam? I don't have time to develop it further now, but maybe it could be added in the future.