The 5th? What do we feel like doing?
I vote pick-a-pack draft or reject rare draft.
Pick-a-Pack
Curious as to how the cards from Ravnica might have played when drafted with cards from Lorwyn and Shadowmoor? The Pick-a-Pack draft variant lets you find out by combining packs from many sets to replicate a Booster Draft!
In the Pick-a-Pack draft format, players draft just as they would during a normal Booster Draft with one change: instead of using three booster packs from the most recent Magic block, players are allowed to use packs from any Magic sets. That means the player to your left could be using a pack each of Darksteel, Ravnica, and Ice Age while the player to your right is cracking two packs of Shadowmoor and one pack of Tenth Edition!
A variant of Pick-a-Pack is Pack Draft. Each player brings three booster packs to the draft, and those packs are collected into a common pool. Before the players draft the cards out of the boosters, they draft the boosters themselves! Once all the packs are picked, the actual booster draft begins. Players can open any of their three boosters first.
Reject Rare Draft
Originally created in New York, this wacky format players a great opportunity to play with rare cards they may have never considered playing with before. Each player donates 45 rare cards from their collections. These cards are shuffled together and dealt out as 15-card booster packs. From there, it's just like any other Booster Draft. Normal Limited rules apply; players must use 40 card decks and may include any number of Plains, Island, Swamps, Mountains, and Forests to their decks.
Reject Rare drafting is a great opportunity to try out cards that have never or rarely seen play in most Constructed formats. It also puts a crazy spin on Limited, which is generally dominated by the likes of commons and uncommons. In the world of Reject Rares, almost anything goes and players rarely know what's going to happen next!
When the draft is over, each player will end up with 45 oddball rare cards, which is exactly what he or she put in. The specific cards will be different, but the fact that the format acts like a rare card swap meet is part of its appeal! (Of course, if the players in your group would prefer to track which cards they donated so they can get the same ones back at the end, you can certainly do so.)