I can't see Fallout 4 being co-op or mmo multiplayer. It just wouldn't work.
But I thought it would be awesome to have some type of multiplayer, specifically for settlements. I want to build a huge fort with traps, walls, guards, and all that jazz. It would be very cool to allow other players to attack my settlement or for me to attack other's settlements. This would not affect any SP settlement, just online leader boards.
Also allowing the attacker to have some amount of men would be nice, it would be an actual attack instead of 1 vs 100.