I don't even think that I've done everything in the game with all of my dozens of characters combined, much less with just one. I just found a dungeon I know I've never seen before the other day.
I will say that in 500 hours though, she has explored every inch of the Cyrodiil map. She's more of an explorer than a quester.
Speaking of which, I just thought of Buffy last night when I happened on a really nice little pond and waterfall fairly high above the Orange Road-- not the one you can see from the road, but another one, further east. It's closer to the Bruma end, just up the hill from a runestone. I could only find one path up to it-- it's got unclimbable hills on two sides and a steep drop to the road on the third. It's right about at the "d" in "Orange Road."
I imagine Buffy's already found it, but I thought I'd mention it anyway.