For Deus Ex (best game ever, and The Nameless Mod rules
) mission mods are separated from the main game. For Morrowind most mods instead add something to the existing game world, through a plugin file that adds things to it, or alters them. This is very easy, but there should be some guides around. And you can use as many mods as you like, the plugin limit lies around 255.
Tamriel Rebuilt is a popular mod project which intends to build the rest of the Morrowind province around Vvardenfell, with cities, NPCs, quests, and those sort of things. It's still in the works, but it has released 2 'maps' (http://planetelderscrolls.gamespy.com/View.php?view=mods.detail&id=7040) so far which add a landmass to the east of Vvardenfell.