Most of the exterior (natural) content for Cyrodiil can Skyrim content or re-textured Skyrim content. There are probably a few areas with specific requirements (near the black march) that don't have similar content in Skyrim. Although some models will need to be created the majority of this work is re-colouring & re-texturing.
Based on some of the early sketches for cities in Cyrodiil, they were't intended to be separate worldspaces. It seems like Bethesda were forced to wall them off for performance reasons. Mods to Oblivion showed that the cities could intact in the main worldspace without killing the framerate, so we should try and keep the cities as part of the main worldspace unless performance becomes an issue. Even if we were forced to put cities into their own worldspaces, developing them in the main worldspace would make it much easier to tie the city in with the landscape.
Each City has it's own style of city walls, building styles, and other unique structures (Lighthouse, Arena, Bridges). These will probably take quite a while to build. It may be best to create re-usable assets - pieces of buildings that can be put together in different ways to make a variety of buildings.
We know what some of the ruins look like from Oblivion, but we'll need to make new models for Skyrim since Skyrim doesn't have these models and we can't use the ones from Oblivion. Additionally, it may be worth changing the designs a bit so that they would actually work as enclosed structures because the ones from Oblivion do not lend themselves to having multiple floors.
Since the Oblivion Crisis the Aldmeri Dominion managed to capture most of Cyrodiil, including the Imperial City for a short time. The Thalmore have probably fortified some areas, but the fortifications will probably be in the Aldmeri style. We need to know what Aldmeri architecture looks like to be able to portray these influences properly.
Anvil: A harbor city on the Gold Coast.
Bravil: A rundown, squalid town on the west bank of the Niben.
Bruma: A town in the northern Jeralls, Bruma boasts Nordic population and character.
Cheydinhal: A town in eastern Cyrodiil whose population has been recently swelled by Dunmer immigrants, due to its close proximity to unstable Morrowind.
Chorrol: A town located between the Great Forest and Colovian Highlands, in the northwestern region of Cyrodiil.
Cloud Ruler Temple: An ancient fortress of Akaviri origin. It serves as a fortress of the Emperor's bodyguards, the Blades.
Imperial City: The Imperial City is the capital of Cyrodiil and the whole of the Empire. It is located on an island in central Lake Rumare.
Kvatch: A city on the Gold Coast between Anvil and Skingrad. It was destroyed during the oblivion crisis but would almost certainly have been rebuilt given it's strategic position on a large plateau, mostly standing castle, and dedicated survivors. It is north of the River Strid in Colovia.
Leyawiin: A city at the southern reaches of Cyrodiil, straddling the Niben River where it meets Topal Bay.
Skingrad: A large town located in the West Weald region, to the southwest of the Imperial City, along the Gold Road.
see also Zero's thoughts on Cyrodiil.
Regions, Places, Areas
The Nibenay Basin
White Rose River
The Great Forest
The West Weald
The Imperial Reserve
The Gold Coast
Sancre Tor: Now little more than a ruined fortress, the city and location of Sancre Tor has been the site of important events since the First Era.