Just read http://kotaku.com/5861752/skyrim-contains-the-entire-elder-scrolls-continent-including-morrowind-and-cyrodiil, and it seems that heightmaps for additional provinces are already in the game.

To what detail, I'm not sure. But this means that you won't have to worry about creating your own heightmaps, or even creating your own worldspaces. Every time you load up Skyrim, you load up the whole of Tamriel.

I saw some pictures of it. I think they're only LOD and just a rough version of it. The shape of Vvardenfell is completely wrong, even taking into account the possible changes from the explosion. The entire place seems to have shifted Northwards a good deal.

There is already another topic on this http://www.gamesas.com/index.php?/topic/1286092-more-than-just-skyrim/

Yep, let's keep it to one topic, thanks