Playing my first 5 solo games with the full game, not the PnP version, i had an idea for a possible 4. Expansion set. I would call theme of the idea "120°" and would construct all hexes according to the pattern C1-X-C1-C2-X-C2 were C means color and X stands for generic. So basically you would have 120° or 1 hex side distance between sides of the same color and between these you would have generic or grey sides. Also you would have the same color combination on the same tile, but mirrored, so on one side you would have orange-purple, and flipping the tile you'll get purple-orange. of course you could also put a non-generice color in between the two, i just thought it would nicer this way and keeps the combinations and therefor the tile count down.
Thanks for reading
Just let you know, I don't plan to reprint MOSD and print more expansion. But thank you for your suggestion and hope you enjoy the game