I've mentioned this a couple of times over the years, but I would really love to see a storyline where we head underground to retake the Asura homelands that were overrun by destroyers. I think there is a lot of potential there. Rediscovering old Asuran tech. New possible enemies that have evolved in the centuries the Asurans have been gone. Maybe even a new mount that can dig through tunnels for fast travel or access to secret areas.
They are available currently, and to my knowledge, are always available. That's one item I believe does not get rotated out, just like character slots or bank tabs.
Personally, I would like to see that guy in the bank in Divinity's Reach who constantly says "I'm rich, you know." killed off. And then give me his money.