The main objective of this idea is to prevent exploitation of the alliance system, for instance by leaving an alliance and getting an instant victory.


Although joining an alliance should remain as it is and instantly raise victory conditions, leaving it should produce effects over time, proportional to the time you spent in it, because the longer you stay in an alliance the more you can profit from it.


So I would suggest something like this:

- If you were allied for 50 turns with another player/AI, when you leave the alliance it will take 50 turns to return to single player victory conditions for both players.

- If you were allied for 100 turns with 2 other players/AI and one player/AI leaves the alliance after 25 turns, it will take 25 turns for the alliance to return to 2 players victory conditions (and 25 turns for the player who left to go back to single player victory conditions) and then, if you leave, 100 turns to return to single player victory conditions for you and the other player/AI.

This system would be even better if the changes do not happen instantly after n turns, but instead slowly decrease over time, starting as soon as you leave (for instance, for wonder victory, 6->5->4->3 obelisks over n turns).


Notes:

- This system can lead to interesting strategic decisions like: how many turns should I stay in an alliance if I intend to leave it and pursue a solo victory? You will need to evaluate how much you gain from being allied, how many turns you'll need to go back to single player victory conditions and if you can do it in time.

- This system is immune to players trying to destabilize alliances by joining and leaving, because if they join (to artificially raise alliance victory conditions), stay 1-5 turns and leave, the alliance will revert to its former victory conditions in only 1-5 turns.


- Into late game, there must be some kind of protection to prevent a player/AI to leave an alliance on the last turns, denying victory for everyone including themselves (except for a score victory). This can be done by preventing any player/AI to leave if the alliance they are in lasted for n turns and there are only n turns remaining in the game. One turn before this condition occurs, a notification should warn the player: "This alliance will become permanent from the next turn to the end of the game", and give the option to stay or leave (which, again, leads to interesting strategic choices).