When a XCom base is attacked by Aliens and they win the fight, instead of simply vanish, XCom team has some time (ex: 3/5 hours, to be determined) to try to take back its base.
The more time passes, the more base facilities are destroyed by Aliens.
The more time passes, the more items at base are destroyed by Aliens.
The more time passes, the more Aliens to fight (UFO Reinforcement mission).
Aliens prisoners join attackers Aliens crew.
The time limit expired, the base becomes an Alien base (Aliens can eventually conserve some facilities for studies, XCom fight in these facilities next attack).
XCom team can try to intercept UFOs with reinforcement troops (UFO Reinforcement mission).
All of this can be optional.
"XCom Can Take Back Base" - "YES" - "NO" - (default "NO")
--> "As Time Passes, Base Facilities Destroyed By Aliens" - "YES" - "NO" - (default "NO")
--> "As Time Passes, Items At Base Destroyed By Aliens" - "YES" - "NO" - (default "NO")
--> "As Time Passes, More Aliens To Fight" - "YES" - "NO" - (default "NO")
--> "Aliens Prisoners Join Attackers Aliens Crew - "YES" - "NO" - (default "NO")
--> "Time Limit Expired, Base Becomes Alien Base" - "YES" - "NO" - (default "NO")
The Aliens can do the same when one of their bases is invaded by XCom team.
XCom team needs to send reinforcement (troop transport) to add troops to the occupied Alien Base garrison.
The Alien base is destroyed (limit of 8 bases for XCom) if Aliens cannot take back their base (time limit expired).
