Being able to play and quest together is fun — and there are some options for Skyrim in that category.
But what would work as a Fallout 4 multiplayer mod?
Maybe I've been playing a little too much MGS V: The Phantom Pain…
But I think base attacking could work.
Perhaps you could send a few raiders to attack a player's camp — and they would be required to defend it. I've seen in the E3 video that you can create a camp and place defences — so why not allow a multiplayer version of that? Able to send some raiders at someone's camp, and if they succeed (say, kill the camp commander), get some caps and scrap.
Another idea I had was something like a Tycoon game. Again, using the creation kind of archetype — what if you could actually send brahmin caravans across to other settlements owned by other players? What if those caravans could be raided by other players? I'm reminded of when Sim City was first announced that neighbouring cities could focus on one type of resource and other players could hook into that resource through trading. How about the same for Fallout 4? Only, you're sending out Brahmin Caravans, and maybe this city has lots of Ammo, and this one has lots of guns, and this one has lots of food, etc. And maybe you could send out caravans trading what you had, with a real-world (or game-time) length between getting your goods. And perhaps if you don't have good security, other players might be able to attack them.
Anyway, those are two ideas I have. I think there's plenty of ideas for mods, what would you like or hope to see?