I have no idea whether this has been suggested before, but how about the trait or ability to be invisible? Not sure how it would work, whether it even can be implemented, but could be a cool idea...

The Battle for Wesnoth..

(some of the units there have invis ability which gives them invis in given terrain => they can't be seen & enemy units lose all their move points for that turn when they 'find' [=move to adjacent hex] them [the invis unit gets revelead by the first unit meeting them ofc])

but .. well.. tbh.. I think we need to get things we already have working. And we can start thinking about expansions (this sounds like expansion ab to me) afterwards..