For a long time people have been wanting to see co op in Bethseda games (mainly TES) and I think it'd be nice to see in Fallout 4.
Now please understand, I am talking about co op, NOT anykind of "mmo" or any of the gameplay that comes from mmo's,
Just simple drop in-out co op that allows a very small number of people (say 2-3) to join you and adventure throughout he wasteland together, exploring, doing roaming around, talking to npc's.
Similar to say, how Baldur's Gate handled MP or Neverwinter Nights.
The host could set rules (IE people that join you can't hurt npc's, talk to npc's, so you won't worry about someone messing up your game), just simple things liek that that would make it work easier for the host and what they want to allow the other people to do while playing with them.