Wouldn't mind a makeshift settlement or two built among or in some ruins but nothing too extravant as a large full blown city with all the amenities.

Hidden cities would be Dwemer cities and we have those in Skyrim.

Thought the title said Falmers for a second.

Now I want a hidden Falmer city. Stop putting thoughts into my head...

A city hidden underwater would be awesome.

Write to Todd Howard, and have it added to the game. However, you wont find it interesting or surprising...since you asked for it.

I've always wanted something like this:

A hidden city that is truly hidden, either in a large wooded area or underground or on an island that is only accessible through some kind of prerequisite, so that when the player gets some kind of item of quest is the only time they will ever see it, otherwise it will just seem like a normal, possibly foggy area. Could be awesome.

Anything less than this would be pretty basic and expected, I would be surprised if there WASN'T something like that.

Then get ready to be surprised.

That would be cool, like if certain forests had camps and villages that were controlled by bandits. And it was dangerous to travel through them. You could have quests to be peoples paid body guards or guides through them.