I've been playing Fallout for a while now, and there are things I would like to do: I need to know how impossible they are before I get too excited about trying to mod.
In Fallout 4, we have settlements and we can build defenses and we must defend the settlements at times.
I find the equipping of the settlers and the various methods of defense intriguing enough that it could be its own stand-alone game: defend all the settlements and slowly improve everything.
That said, the behavior of the system in Fallout 4 as it exists now is... well, sub-standard for the purposes of this kind of mod.
The AI allows NPCs and enemies to simply pass through walls / spawn inside of walled areas- they can also basically teleport around and up onto things. It's understandable that it was built this way, but that kind of thing is what would need at least some tweaking for a settlement defense game to pan out.
Better management of settlers and resources is needed, too- some new panel in the pip-boy menu?
Also, if I give a settler or NPC the chameleon armor presently, they do not pop into stealth mode when they crouch. Exactly how do the stat and perk benefits apply to NPCs?