One of the most requested features from Fallout 1 was multiplayer. It would have allowed you to import a character and play a LAN game against other players. Combat only, no questing, no character development, just a simple bar fight. I'm going to take a stand and say that multiplayer in FO1 would have been a mistake. Because it wouldn't have been what people were expecting.
Expectations are a powerful thing.
You just can't add a large gameplay feature like multiplayer to an existing game and think that it will work. You need to design it from the very beginning. I can't think of right now. Anyways, multiplayer was always part of the Fallout Tactics game plan. We discussed it very early on.