1) Bring back the old hosts. Tell me that it wouldn't be absolutely thrilling to have a big twentieth anniversary special where Craig Kilborn and Jon Stewart team up with the new host to tackle a really big news story that none of them could handle alone. There'd be tremendous potential for comedy there, as the three hosts bickered amusingly about whose style was best suited to the situation, along with some witty meta-commentary on how the show has changed over the years. (Maybe have Kilborn deliver a quip like, "You've redecorated, I don't like it"? I'm just blue-skying here.)
Maybe even, for extra dramatic effect, we could find out about a "lost" season of the series, where a comedy legend was the anchorperson? I bet they could get Bill Murray. He'd absolutely kill with the way he kept suggesting that the "new boys" were too young to do proper comedy. Of course, in order to be big enough to bring all four hosts together, you'd need...
2) Returning villains! Really, one of the things that 'The Daily Show' has always been famous for is the monsters. Now, obviously you can't bring back some of the really monstrous villains--Saddam Hussein and Moammar Khadaffi are dead, for example. But there are still a lot of good B-list villains to choose from. Dick Cheney's still out there, and George W. Bush has been laying low for a long while. Ken Starr hasn't been seen in almost fifteen years, and a lot of us still have fond memories of his time on the show. We could see a huge gathering of these classic baddies, led by maybe Bill O'Reilly or Glenn Beck? It'd be really exciting, especially if they...
3) Incorporate the spin-offs. After all, if it's the kind of crisis that forces all the incarnations of the series to work together, it's got to be big enough for the Host's old allies to return. Picture them standing side by side, Jon Stewart and Craig Kilborn and Bill Murray and (host to be named later), overmatched against the Fox News anchors, when all of a sudden Stephen Colbert shows up! And just behind him is Jon Oliver and Larry Wilmore, and Steve Carell! All the Classic Series correspondents, teaming up with the new blood to help out the Host of the Daily Show in his...er, their...hour of need. It would be breathtaking.