DLC that gives the option to skip the introduction sequence. There are already mods that skip this for Fallout 3, New Vegas and Skyrim. I'm actually surprised they didn't offer this in the base game.
DLC that adds a hardcoe mode.
DLC that removes player voice acting and brings back the Fallout 3 text dialogue system. Already been requested but this is the only new feature of the game I really don't like. Bethesda could probably profit from this quite a bit, as almost every player made quest mod would require it.