... would have loved something like Zerg Creep in Starcraft -- only in a more ephemeral sense: The corridors between settlements become safe passage, and the more settlements you make/maintain, the more the influence spreads out from those nodes and lines. Couple that with non-infinitely-respawning Raiders and monsters, and maybe I'd feel like I was accomplishing more...
Sadly, come end game, they need to be able to have the various factions still act like everything is on the line. and if you could literally rebuild the commonwealth into a safe, happy, well defended, nation... then I don't care how many synths the institute has, they're not a threat. And even the force of Brotherhood on that airship couldn't stand against the commonwealth on their own.
I do also get why they don't let you make everything super safe, and unite everything in 4, while the game map is a certain size, it's meant to replicate a larger world, that would be MUCH harder to tame, given the number of "settlements" and their scant populations. I mean, concord and Boston are a 15 minute walk apart in the game. it'd be a 6 hour hike in the real world.
That said, what I WOULD love, is a game where the
purpose was a lot like you said. A "Fallout: Vault City" or "Fallout: Shady Sands", where the goal is that you are rebuilding society. the villain isn't some big baddy that needs a bullet in the head, it's the fact that the world is just a ****** place. You'd start with a home base area, be it a Vault, or a town, and your job is to accomplish things out in the wasteland, like securing large sources of materials like clearing a building or neighborhood, which allows your fellow residents to go scrap and scavenge there. The wider you spread your influence and your satellite settlements, the more you're able to institute your own "minutemen" style protection that actually WILL keep areas safe. add an option to just upgrade areas assassins creed style ("I don't care where the new fountain goes, just build one"), or to do it fallout 4 style, and that'd be a hell of a game.