Reader Elstor, who sent us this question the other day, had some ideas himself, such as Oshu'Gun, the giant diamond mountain in the middle of Nagrand. It's honestly a good idea. Unfortunately, the Horde is the only faction who gets quests to head into the middle of the mountain and find out its true secret, as well as gain a valuable insight into the nature of the Naaru. Fleshing out Oshu'Gun as an instance would have provided some great lore insight into the Naaru (and maybe even the Horde) that the Alliance is sadly missing, and would even be an opportunity to further develop the split between the Kurenai and the Mag'har.
For my own part, I would still love to see a third Razorfen dungeon that would hint at all the possible connections and convolutions between the Grimtotem, the Quillboar, the Forsaken, and the Scourge as per this Thousand Needles quest. Alliance could even get in on it through, say, a letter dropped at a Grimtotem camp in Dustwallow. I was reminded the other day that Grimtotems do have that new skin, and I've always wanted them to take center stage more than they do. I continue to be convinced that Magatha is essentially the Horde's version of Lady Prestor. Plus, I'd love to see more background on the Quillboar and the desperation that drove them to ally with the Scourge as the Horde slaughtered them and took their land.
I'm sure there's tons of areas and quest lines that would feel a bit more epic with a nice 5-man instance attached to them. What are a few you might have wanted to see?