What to do in Skyrim when you beat all the questlines?

There are dozens, if not hundreds of side-quests to complete in Morrowind, including but not limited to working your way up and taking over leadership of all the different guilds. If you feel the game is over after you have completed the main quest you are missing a lot of what Morrowind has to offer.

The expansion packs are great too. Both Bloodmoon and Tribunal have their own main plots, as well as multitudes of side-quests, new creatures, buildings, items, etc. If you are using the PC version of the game it might also be worth learning to use the Elder Scrolls Construction Set. With this you can create your own caves, towns, characters and objects, edit dialogue, write scripts to control advanced behaviour and much more. There are many such user-made "mods" available for download from the various fan-sites as well as from the official Elder Scrolls website. Many of these add new quests. Some even add entire new continents to the map!