I'll be having a game of L4D2 tomorrow, taking another stab at the Mutation. Find me on Steam ([DFC] Zai) if you want to play along, likely on Normal (Normal was plenty difficult the other day, even for people who normally play much higher; but then, I don't think any of us used mics). Hit me up whenever I'm on, and we'll find enough people to play with. I'd rather not make another group event, though, as some might consider it overkill for so few people playing. I also don't want to commit to a specific time tomorrow. If I can't sleep again, I'll see who's on at whatever time that has the game.
I think I'd like to play either The Passing or The Parish. The Passing for the easier finale due to having 4 extra Bots that can't die (I think), and The Parish for a challenging finale. Making it to the end of the Bridge with pairs of Specials coming after us will makes Jockeys and Chargers more than just a double threat since they can incap Survivors by running off the bridge. I'm not sure if that's in bot AI, though, so we may not have as much to worry about as I think we will. We may do both, warming up on The Passing and then doing The Parish.
I think this Mutation would be even better on Versus or (better yet!) Realism Versus (though that might be too much of a challenge). And it could even keep the normal 4v4 (rather than having 4 people sit out each Survivor round in 8v8) by having 4 Specials be bots. Alternatively, it could seem like even more of a heavily modified mode by having 12 people play at a time, with the Infected continually playing round after round like normal Campaign, instead of playing the campaign twice, and then going back to lobby to switch. Though unless any of these ideas are implemented in a fashion that includes Campaign-like restarting (not likely due to eating up loads of time, though this would have the least effect in my non-doubled-up idea), the high difficulty of this Mutation might be stripped away somewhat due to not having to restart, similar to Realism Versus.
Regardless, I'd love to see this Mutation return as a normal game mode (or at least an option when starting a campaign). It really was quite fun the other day.
[EDIT:] Heh. I said "4 bots" in The Passing that don't die without thinking. I meant 3.
[EDIT2:] Judging by what I've seen on the Steam forums, I'm the only one who enjoys Hard 8. =/