If a player gets disconnected or leaves, let one who just queued up get the option to either join a fresh game or one that is missing people? And maybe give them bonus experience and/or veritanium for filling the empty spots, maybe even a lockbox.
If a Fireteam member is killed during the round and instantly leaves, a new member can only join once the Fireteam has called in reinforcements.
From my experience it’s usually the predator that gets the “Signed Out” at the start of a round, once the predator is gone it’s not very fun to play anymore.
When new players join the round a message should pop up on the screen saying something like: “A new Predator has entered the hunting grounds!”
Or is this a bad idea?

