Maybe Cubecraft could firstly try making a /rejoin for tower defence, making it not work when in a party, when a set time has passed and if the player joined another game of Skywars, Eggwars or whatever.
When a player has left the game said player should not get any coins or XP and towers of his should be replaceable.
Making a somewhat of a Beta version of this command for tower defence could shed more light of how long it takes to make such a command, how this command should work and be implemented.