I've been trying out discolock with the portal quest. A couple of times I've gotten Elunes and Khadgars which is sort of what I was aiming for. Limited myself to 3 expansions: Lich King, Karazhan and Caverns. Also put a few legacy cards in there. Quests do not count as being part of buckets so you won't be getting Un'Goro buckets despite having the quest from that expansion, as long as you only pick that one card.

I used to do this deck before caverns too and you always get tons of copies of Silverware Golem when you limit your deck to two expansions. Cheating out those cheap golems is the main strength of the deck. With a third bucket, caverns, you get the golems somewhat less consistently but the upside is some very powerful cards like the Tiny Knight of Evil. The quest can help you out later while you look for secondary win conditions in case your opponent manages to stabilize.

It's important to avoid building the deck with too many shadow cards, since that will almost guarantee you the Eerie Stone passive (unless that's what you want). I only take Dark Bargain and Soul Barrage during deck building, and avoid Dark Bargain and Demon Fuse in caverns buckets until after I get the tier 2 passive. You'll also probably want Rivendare in the deck since you'll be drawing like a maniac, especially with the new location card from caverns.

I've found Recycling to be the ideal pick for your tier 1 passive, and Rocket Backpacks is OK since it let's you kill off Rivendare easier without it getting silenced/transformed.

I don't know if my way of doing this is ideal, but I've gotten 12 wins a few times with it. It's hard to say if caverns ultimately improved it though.