"Feature" doesn't mean any new concept, only terrain features (forests, flames, etc)
The fitst "feature" you suggested is basically thr same thing as the Forest stealth promotion. This was a recon-only promotion that required combat2 and woodsman1 and granted invisibility to the unit only while it was on a tile with the Forest, Ancient Forest, or Jungle features. This was present in all the pre-BtS versions of FfH (at least since I discovered FfH .16 a little more than a year ago). I will be greatly disappoined if it isn't brought back.
There are already units in the game that can detect invisible units (Hawks and Sand Lions, for example)