In order to keep the dungeon crawler feel I don't feel some specific dungeon would suffice. This isn't WoW (thank everyone there is thank for that).
I think a type of scenario or repeatable objective would be nice though. Similar to the attack going on outside Harrogath, only bigger.
Endless (seemingly) waves of monsters booking it towards town. The group (players) must defend the town. The mobs speed at which they come are based on both combined character levels and the speed in which everyone is killing them (as to keep a steady not to fast, not to slow flow of them). Over the course of the event, these mobs will give exp equivalent to what one would get spending the same time in baal runs + x% to make this the preferred method over Baal runs (or reduce Baal run exp).
After X mobs have been killed (or a max time, say 10-15 minutes and a minimum time (to limit max xp/hr) of 5 minutes) a 'boss' will stroll along. His spells and abilities will be geared towards requiring a good variety of classes, inspiring organized private runs, but would still allow the public games to happen, just not as efficiently (which is how it should be). This boss will have varying mods (random immunes, etc) and will have a similar loot table to Baal. The players only have X minutes to kill the boss before he (despawns/explodes/becomes Will Smith/whatever would be cool).
Of course this is Diablo 2. The town would never become overrun. If things get nasty everyone can retreat back into town to regroup and head back out for more. This would allow the non elite players to group up and still level efficiently, rather than the current method of leeching off bots.
That would keep me leveling all sorts of classes/builds.