I have an idea for a super cool feature - the exact opposite of how custom maps are made. Have a tool that takes heights of each tile on the map and converts them into a 16-bit .png file.

Why would anybody want this?
As i've been making a map for quite some time now, i find it very hard to create some fine details just in a graphics editor, where differences between shades are so small that you can't even see them. And let's be honest - most maps that are made mainly by "smooth" tool applied on some real-world terrain have all their hills round and edges smooth and it all looks kind of unnatural.
So i was thinking - if I could upload the rough map onto a server, make the desired terrain changes in-game and then export it all into a new height map, which could be uploaded again to generate the terrain "naturally", it would be just awsome.

Since it's not that hard to alter server settings so that you can for example have paved roads with 10-meter slope and you can pave on top of an already paved tile, this tool could be practically used.

Then of course comes the problem that people might just upload some official FeudalTools map or a map created by someone else and make their own height map out of it - but this can easily be prevented by either setting a password when uploading a height map, or only allowing this feature on maps uploaded by the same user.

I somewhat agree with this, as I continually run into the problem of the subtle change in photoshop turns out to be distinct terrain lines in feudaltools. I've tried extensively to work it otherwise, to no avail. It has to be a base terrain map as you said, with only a few subtle changes or else it turns out all lines.

This is now implemented with the 2.2 update
https://nyuton.net/feudaltools-update-2-2/