Apparently there is a mod in the works to make all the world outside of Skyrim playable.
Quoted from Kotaku commenter -Silver-:
"For those of you interested in new provinces outside of Skyrim, there's a huge collaboration of modders working under the banner of "Beyond Skyrim" to create a huge open world space of Tamriel, and fill it with 100% lore accurate geographic regions, cities, and quests.
Morrowind has a team. High Rock, Hammerfell, Elsweyr, Cyrodiil, and Valenwood do too. The map is to Skyrim's scale (so monolithic), and completely open. So you could travel by foot, horse, or sea, from the South West of Valenwood all the way to the North Eastern tip of Morrowind's Telvanni islands without a single loading screen. You can see the heightmap in the attached image, and for those of you who don't know Tamriel very well, Skyrim is at the very top center:
http://www.tes.altervista.org/mappagigante.jpgCheck out the forums here if you want to lurk - plenty of WIP screenshots, concept art, and discussion:
http://www.darkcreations.org/forums/forum/119-beyond-skyrim/"