I think it would be pretty fun if players will be able to join someone's waves game as an enemy. I have two variants how it can be realized
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The "intruder" can move like a spider and use weapons, but cannot use a web and other perks that the player has
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The "intruder" spawns with a random weapon that can't be thrown or left out of ammo. For example, he would have an infinite shotgun, but it will have a really long reload time. He can use a web, but not player's perks