I searched this up and nothing was found that related to my idea, so I figured I should pitch it. Marauder Round would let players do no damage to props or props would have increased health (optional), player health would also be increased (or gun damage lowered). Players would be able to deal damage to eachother and heavy props would be granted buoyancy, although prop limit lowered to 25. (Optionally) Additionally, this round players get the chance (if they acquire bonus item(s)) to raise a random player from the dead whom would deal far less damage; still a fun way to keep players interested. Undead players could come back as http://steamcommunity.com/sharedfiles/filedetails/?id=305800990&searchtext=grim+reaper (whether they permanently gain the model or temporary is up to you)
This seems a little complicated for flood, like reviving dead players, doing damage to players not props (just build a wall out of something cheap and big, health of a prop won't matter.) Seems like an intriguing idea besides that stuff listed about and what Rick said.
Could make prop damage possible, only it would have to be less. If you want complicated we could make the air poisonous and the water safe=heavy prop boats. xD