What do you think......
Add an option in PvE custom lobby that allows one player to be able to join mid game as 'enemy'.
If the option is enabled, the player who wants to be the enemy has to pay a large amount of coins before joining.
After joining the game, the player will have an option to choose to be a normal size AI ship or a group of small planes, with limited lives, probably fly in third person view.
The player-controlled AI ship can be identified by other players' spyglasses.
If the player managed to stop the other people from winning the game, he will gain a much better war effort to his faction.
If the player did not success and run out of lives, other players will gain some extra amount of war effort and share all the coins from the enemy player after winning the game.