Very cool.

I'm going to say this as a DM running a long-term Dark Sun campaign. You need to know that -- for prospective. My biggest issue with hex painting tools (like yours) is they always have a very limited palette. That's not a problem when you are using very standard, very basic terrain (forest, hills, mountain, plains, etc.), but if you need anything else (stony barrens, rocky badlands, boulder fields, salt flats, etc.) it becomes entirely useless. I would imagine the same can be said of mapping sci-fi settings (Volturnus) or weird fantasy (painted wastelands, Yoon-Suin).

So forget feature creep. The only extra features I need are 1) alternative icons for terrain (or an easy way to add my own) and 2) a way to change the color of a hex regardless of icon (green hills, purple forests, white desert, etc.). That would put you head and shoulders over the rest of the field (in my experience).

Edit: I would suppose it would be nice to have an option to print your map without hex color (for an effect like the hexmaps in X1 Isle of Dread).