I loved running around in Dwemer ruins in Morrowind. It didn't matter what quest I was on, if I saw one of those, I was off to stick my head in it [and occasionally get it ripped off...
].
The Ayleid ruins weren't bad in OB, but they still didn't feel the same. XD
I'd also love to see Winged Twilights. Those were my favorite MW mobs, and when I made myself a random house, I even made a peaceful Twilight that would wander around.