The sixth man could really be just about anyone, because the person being added doesn't really have to be, and likely won't be, a main event contender. Let's face it, in all likelihood, had Jericho remained in this match (and he could still be re-inserted mind you), he in all likelihood wasn't going to win, as it's been widely reported and speculated that he's taking some time off.
I could see Darren Young being the sixth guy. Obviously he's not ready to hold the world title and he really isn't in a position to even contend for one, but as I said, the sixth person will likely not be in serious contention for the belt anyway. Young could come in as a face and cost Barrett the match. Or he could return to heel status, helping Barrett and re-joining Nexus, especially with the loss of Sheffield lowering their numbers.
I could see John Morrison entering the match in Jericho's spot. He probably won't win or even get serious consideration either, but for all of the reasons I already stated, that's irrelevant.
I don't think it will be HHH, I don't think he's physically ready at this time as I understand it. As well, he apparently just got appointed to some backstage role/promotion involved with creative and while I don't think this means he's done, it in all likelihood means he's done for another little while. No need to rush him back. Of course this could all be a swerve as well.
They could just leave it as is, and only have 5 guys. As I recall (and I could be mistaken), didn't something happen prior to their last 6 pack challenge costing someone (possibly Foley?) an opportunity in the match, and they proceeded with just five? They also could insert any random guy (Bourne, Ryder, whoever) just for filler.
Here's what I'd like to see. The Miz drops his US belt to DBD earlier in the evening. Some would argue this makes him look weaker (I don't agree with this personally if it's done properly). So the Miz inserts himself into the 6-pack challenge and makes it known upfront. In other words, rather than being a cowardly little shit like all of the previous MITB winners, waiting until the champ has been beaten down and incapacitated, and then exploiting the situation (like the "ultimate opportunist"), have the Miz come out after losing to Danielson, saying even though he lost, he's still Awesome!!!!, and that he'll prove it tonight in the 6-pack challenge. Have him participate in the entire match with his intentions known upfront, and have him win the title in this manner. No longer has to worry about looking weak by losing to DBD, and gives himself instant credibility by winning the title fair and square against the top guys on RAW. This would enhance his push far more than having him come out after, for example, a Nexus beatdown of someone to win the title in a cheap fashion, and certainly is more beneficial to him than having him lose his MITB briefcase altogether.