Noted. Still as I mentioned on #Erebus, since it is easily implemented, it could be used in at least these three examples with moderate success:
A) An atmospheric unit could be used to scout foreign closed borders, very safely, without being able to see the actual units.
B) An underground unit could be able to make use of peaks, building tunnels and underground improvements.
C) A submerged unit could hide under-water, travel under sea ice, build underwater improvements.
I also envision each of these to be given separately to different civs. A) to the mechanos? B) to the Khazad or a new Dwarven civ, C) to the Bezeri or Isenj (whichever it fits with better)
The system is not perfect. In many situations the underground/submerged/atmospheric unit will be able to see units because other units the player owns can. But that is not the point. The point is that in most cases the units will be able to occupy the same tile as an enemy and not be noticed. For the most part enemies will not be able to see these types of invisibility. Units with this effect could become bNeverHostile when they are in a different altitude. Since they aren't meant for direct combat while in these modes it works. Also when the unit is isolated from units which can see surface units, those units will be invisible. In other words if a Hill Giant is traversing a large mountain range and the dwarves are mulling around underneath, as long as land based units are not able to see the Hill Giant already, the underground dwarves don't gain any visual advantage. In fact, the dwarves under the mountain and units underwater should gain a vision nerf. Perhaps restricting them to the tile they occupy.
Also spawn groups could be added which occasionally spawn in the same "altitude" that can cause issues like pillaging (I'm thinking Balrog like units for underground and sea serpents for underwater).
Here is an idea for dwarven peak improvements:
Under-home: +1
grows to Under-village in 20 turns (must be worked)
Under-village: +2
grows to Under-town in 40 turns (must be worked)
Under-town: +2
, +1
this leads to a +2
, +2
, +1
tile in 60 turns with used worker turns and time invested growing the tile. No artwork is necessary because the improvement is essentially underground. Only the random spawned "Balrog" type units would be capable of pillaging them so they can't be too strong. They could also require a new tech which requires mining, no ideas for names though.
No ideas for underwater improvements yet (though I envision being able to build them under ice as well)