Dibella's Watch adds an entire new continent to Oblivion. It comes complete with a major city, castles, towns, villages, farms, monasteries and all the good stuff that makes up a Medieval landscape. There's also fighting to be had and quests to be made!
Here's a little special something for you. This shows an area I'm working on right now. As you can see, the creation process in the Construction Set doesn't look too thrilling. On top of that, the CS is buggy and has as a favourite pasttime to crash on you! The image to the right shows the same area in-game though. It IS worth the effort.