While it seems that it is possible to add additional terrains to be randomly chosen for Base Defense:
alienDeployments:
- type: STR_BASE_DEFENSE
terrains:
- XBASE
- MILITARYBASE
The issue afterwards is how to assign map blocks to base modules since the code seems limited to only allowing 1 map block for each module, meaning that only 1 terrain/map block can be defined for each module:
facilities:
- type: STR_ACCESS_LIFT
mapName: XBASE_00
Despite my testing I can't seem to find a way to have both XBASE_00 and MILITARYBASE00 as the Access Lift, depending on the terrain. Any pointers on this issue?