As title suggests, I want to see a Tower of Druaga style campaign added to the game.
Basically, a huge (HUGE) tower. Many many floors of increasing difficulty. Say, from a few kobolds on first floor, to epic dragons and pit fiends, titans, etc on the top floors (so that they're almost impossible(slightly possible, but very unlikely, even with perfect strategy) to beat). Either add some new epic loot, or possibly make the higher levels drop epic loot (scrolls only ) from other content packs.
Any level you beat you can go back to any time you want. But you must beat a level to advance to the next one. So if you played level 1 - 10, you can either do 11 next, or go back and do 5, and then 6, and then go to 11. This way people won't be forced to find people on the same level they are on.
Make it in it's own zone as well, complete with vendors, taverns, auctioneers...
I think something like this would be extremely enjoyable. Not to mention addictive.
*edit* Almost forgot. On the top level (if anyone were ever fortunate enough to get there, let alone beat it), there could be a shrine that could grant a special feat that adds +1 to all abilities (like completionist, but not as good, and stacks with completionist)? (: Gotta have some kind of incentive to beat it. Not that you ever will.