Here is a simple idea for Skyrim Modders.
It is just the very core idea, then everyone can develop from here.
Underground rivers crossing the land of Skyrim.
Holes on the ground will lead to underground rivers, taking you to different destinations.
You just need the detailed map of the wells or holes and the direction of the water flow.
Basic Ideas can go a long way!
ps: just contact me if you like the idea and want to make it ''reality''.