Hi,
Maybe somebody had this ideea already, but here it is: a Mined Field terrain improvement. Enemies won't be able to see it until hit it. Traveling trough Mine Field would damage any land based unit. Can be removed only with specialized Demining Engineer unit.
Exhanging maps also reveals other players Mine Fields positions.
Eventually, equivalent sea mines.
They tried this in CTP but had to take it out at the last minute because the AI couldn't handle the capablility correctly. I don't expect it will be in this time either.