Since I have my hands full with another expansion, I'm going to throw this idea out there for anyone to grab. Maybe its no good, I don't know. What if there were clear plastic tiles that depict an underground element? The plastic tiles would be played over the top of any other tiles. You would draw a plastic tile every time you draw a tile with a Catacomb icon, or maybe just have the choice to choose one tile type each turn: paper or plastic
. This way, you could literally create a connected cave network beneath Carcassonne. There could be mining, underground cities, railroads, & waterways, grave robbing, secret treasures, cave monsters, new ways to connect cities, etc. There are many directions you could take the idea. Crafting tiles would be interesting, probably using overhead projector sheets.