It would be nice, but it would be too much effort to re-create cyrodiil in the new engine. Sure, they could use some of the art assets from Skyrim - people, faces, armour, some of the houses and structures.... but they'd still have to design a new land-scape, and new art to fit in with the much less snowy setting.
I can imagine them putting in some DLC in which you explore a city - maybe a small island much in the same vein as Oblivion's "Shivering Isles" DLC - which was awesome by the way. But like Fallout and Oblivion, if there are new areas to explore as DLC, you won't be able to walk there - you'll have to go through a cave, or a portal or a place that will transport you there automatically. It's just much easier that way.