That would solve the entire VATS question in one fell swoop - a multiplayer Fallout spin-off would just have some other mechanic than what basically amounts to bullet-time, while still serving some of the same purposes. You'd also get the benefit of a game that would be built from the ground up to be fun to play with more than one person, as opposed to a game that's built with a single player in mind. (I mean, it seems like if you just jumped into a friend's FO3 game, you'd be watching the other player go through lots of dialogue and the like that has no relevance to you.)
I'd just as soon see a completely different spin-off that was made from the ground up with two or more players in mind; and with quests and the like that were designed to be explored by more than one person. Even dialogue could be incorporated to take into account that another player is with you (or an NPC fill-in, of course.) A really clever game designer could probably even come up with some sort of system in which both players could interact equally with the NPCs.
Anyway, I'd just rather see something totally new than trying to fit a multiplayer component into a game (even the hypothetical Fallout 4) that's nominally made with just the one player in mind.