I had this idea in another thread but wanted to ensconce it in its own Suggestion
I admit this is blatantly stealing gameplay from Torchlight, but hey - its an idea that's proven to work, and its a whole new dimension of repeatable content for relatively low investment in resources, compared to handcrafting quests and expansions...
These would be essentially linear dungeons - probably set in tunnels in The Underdark/Khyber/Stormreach Sewers (depending on level). All these zones are supposed to be immense to the point of unmeasureability, after all. They would be constructed by stringing together premade "tilesets", and seeded with random appropriate monsters, occasional random encounters with chests (like Explorer zones), and occasional scripted events (again like Explorer zones).
You would get minor XP from the scripted events, and also for completing Slayer deeds that would reset every time you played through a Random Dungeon - ie, when you enter a dungeon, you get an XP reward for hitting 100 kills, 250 kills, 500 kills, and so on, and if you recall and rejoin, those rewards are reset, basically like Optionals on quests.
The CL of mobs and chests would increase as you went through the dungeon....say, every 4 or 5 tilesets, the CL would tick up, so that eventually you've gone "too deep" and have to recall out or get overwhelmed, but the longer you could manage, the better rewards.
I think having this open ended style of gameplay would be a nice complement to the structured, goal-oriented gameplay of quests and raids. It would give players who maybe don't want to commit to finding a group or running a quest an option to "do something" that still, in a small way, advances their character. I don't expect that these would ever replace quests in terms of XP/min or chests/min, only that it provides a little more "immediate gratification" than having to spend 15-20-30 mins before you get your lump-sum XP reward and your big chest at the end.
And it provides a little arbitrary challenge for those who like to measure their characters - see how deep you can go! Maybe even have some special "EE raid level" encounters that only appear once you've gotten to EE raid level CL zones, so you have to fight your way down there, and survive until you find them, for whatever special rewards they can offer. In a way, a bit like Subterrane.
Plus, once the system is in place, all you need to do to expand it is design new tilesets (or steal them from existing quest construction), plot out spawn points, script a few new events, and plug it all in.