Would be interesting, but I can imagine a lot of people wouldn’t like anything that messes with their strategies. As long as it’s not forced on every server, sure, I don’t see why not.
In L4D2 you have to specifically start a game in mutation mode. The normal game modes are still available. I played a couple of these mutations back then and most of them were pretty fun and a nice distraction from the standard game play.
With DB and its current low player base there is a certain risk of fragmentation, but other than that I can see it working. Provided SD (or the community) can come up with fun ideas.