These three change how the whole round plays.

Sleeper Cell
Some of you are secretly infected. Work out who.
8+ players
About one player in five is a Carrier. Everyone looks the same: the same skin, plain chainmail and no cosmetics. There are no name tags, everyone else shows as "Someone" in Tab, chat doesn't show who's talking, and voice chat is muted.
A red or green bar at the top of your screen tells you which side you're on all round. Carriers can sniff out nearby players with F.
Carriers can see each other and try to kill everyone else. Anyone can hit anyone, so the survivors can fight back. If you kill someone who was clean, you flash red for a second.
When someone dies, everyone is told what they really were. The dead watch until the end.
Survivors win by killing every Carrier or lasting until the timer ends. Carriers win if nobody clean is left.

Patient Zero
One Mother Zombie with 40 hearts. Bring her down.
8+ players
There's only one Mother, and she's a boss. Every 12 seconds she uses another class's ability.
She can appear right next to a survivor, and she can stop knockback for a few seconds.
When she goes down, every survivor heals 3 hearts. Whoever lands the last hit gets 15 cookies.

Role Reversal
Survivors chase. Infected hide.
8+ players
A few players become the Cure. Every Cure gets the same kit: rank 10 armour, a stone sword, a bow with 3 arrows, a sticky grenade and 2 apples. Everyone else turns infected right where they're hiding, with some food.
Every infected the Cure takes out joins the Cure. A Cure who goes down comes back.
Scorestreaks swap sides: the infected earn the survivors' scorestreaks, and the Cure earns the infected's. In the last minute every infected glows, so there's nowhere left to hide.
The Cure wins by clearing the map. The infected win if any of them are still standing when time runs out.