I think it would be a cool mod. Would you alter the spawn points as well to adjust for the layers?
Yeah, I would envisage the aliens spawning on the surface level. But everywhere and in large numbers.
Well the question is how you wanna do this technically. I suggest vertical Map overlaying.
Do you want to rework all Base Modules? (i guess thats too much work, because it also would include the vanilla ones).
That's what I planned, because of technical issues. But, as you write below:
It would be more practically if we could layer the maps on top of each other.
Level 0 and 1 would be XCom or Alien Base modules, and above the Groundlevel which could include military buildings for xcom bases and Terrain according from the Terrainpack for aliens Bases of course we would need to match the Entry points, e.g. Hangars, Access Lift and the Alien Base Entries to wards a maptile which would allow the soldiers to take a lift or enter somehow.
Actually, that was more or less my initial idea. I wanted two different sets of buildings: overground and underground, as well as a toggable view of the base (surface/underground).
However, I found nobody willing to write the code, and I sure am not prepared to do it myself, so I aborted this idea for the normal map, albeit 10 levels high or so (lowest two: the underground base; next one: dirt and lifts; next one: surface; the rest above: air and buildings).
I guess that way it could be done and we did not have to rework all maps. But create a few new ones and use use what is already there.
But just a Suggestion.
A good one.
* When adding surface buildings why worry about new tiles when you could use tiles from terrorsite warehouses, Hobbes military installation, and the existing base facilities. Why not build a tileset from those and then worry about making new tiles IF you really want them.
Yeah, I think I said I would like to use Hobbes' base.
As for the two-stage mission, I'm not really convinced. I'd prefer standard, multi-level missions, even if they could be a bit tedious (though we could make a few more entrances).