Along with co-op there should also an infinite multiplayer world where it doesn't load when you open doors.(along with coop too) And if you die on coop or multiplayer, you could be revived by a defilibrator or spawn in a certain hospital or home.I know a great story for coop but you could add your own if you want.

We already have an http://www.gamesas.com/index.php?/topic/1112031-fallout-multiplayer-discussion-thread/ for all multiplayer discussion pertaining to Fallout. You're welcome to post your story idea there.