I've seen the few random requests for Fallout multiplayer, and while I believe Fallout: Tactics is the closest we ever came to something along those lines, multiplayer will probably never actually 'belong' or 'fit' with the sole-survivor post-apocalyptic genre that is Fallout.

That being said, the settlement 'mini-game' (as some people have termed it) could be used in a unique sort of way. I was thinking along the lines of the 'dungeon' mobile phone games, where a player creates their little fort or whatever and then another user attempts to 'lay siege' to this fort. While it isn't full multiplayer, per se, (at least the few I'm relating this to, i.e. Dungeon Hunter) it still involves another user's creation and a sort of 'guardian' that is created in that user's likeness and defending the fort along with the other minions.

Imagine building up your supreme Castle settlement (artillery and all? maybe..), tasking which settlers are where, and then uploading this to a friend to storm. I agree, still alot of external coding to be done even on this type of limited-degree multiplayer, but it was worth a pitch. Flame away.

It's an interesting idea, and when the CK comes out is entirely do-able. It would just need such mods to be tagged with a 'take the fort' marker.

What would be tricky, though, would be taking a settlement created in-game and exporting it as a mod that could be added to another person's game. Not impossible in theory, of course, but it would need someone to reverse engineer the savegame format and figure out how to identify the relevant records and re-create their .esp equivalent. Depending on how the savegame format may have changed, I'm not sure if that would be trivial or complex.

If people are making such settlements in the CK then they could get far more creative about the layout (you could, for instance, have a whole vault or large tower block for players to attack) but it may also be slower and harder to create these than using the in-game settlement builder.

I think that mods where you get a pre-built settlement to defend could also be fun, as you'd have the ability to move around the defences and add more. Something a bit more like classic tower defence games than the 'defend settlement' quests. Heck, if you were defending against waves of synths and robots, you could have an option to race against the clock between waves to loot and scrap them for parts to build more defences