Old Post Incentive to spare the Quincy Survivors

There doesn't seem to be any benefit to letting Preston and friends leave Concord; they move to Sanctuary and spend the rest of their days hammering at wall panels and complaining, waiting for your input. Do you think it's possible to have them build a city themselves if left alone long enough, allowing you to raid it later?

This would require that the player start the game unable to use the Sanctuary Workshop and have to unlock it by either killing the Quincy Survivors or helping them as per the base game. I can certainly understand why you might not want to lock the Sanctuary Workshop, as it is the first settlement that players get access to, but I believe that Red Rocket being just across the bridge reduces the impact on the game's progression. I've made a handy flowchart to illustrate how I imagine the system would work.



The lock on the Sanctuary Workshop can be removed in three different ways. The first, most direct method is to to simply kill them in the museum. The second method is to progress through When Freedom Calls as normal, rescuing the Quincy Survivors and accompanying them to Sanctuary, unlocking the Workshop when Preston directs you to assist Sturges. Finally, if the player does not meet up with Preston at Sanctuary within a certain amount of time (for example, 48 hours), then a city plan is built and the player will see it when they next return to Sanctuary. To gain control of the settlement, the player would then have to complete The First Step for Preston and become General of the Minutemen.

I'm not sure about from a technical standpoint, but one hurdle you'd have to overcome is how to convey what's happening to the player. Which means there'd need to be some dialogue. Not difficult on the Raider side, since you can add an event where Jammer finds out and has a "Hey boss, you remember those settlers you let go...?" conversation. That's easy enough, but it leads into a fourth branch not on your list where you have to go conquer the settlement as normal.

Where things get tricky is when it comes to future interactions with the Sanctuary group once they're settled in. You'd want some kind of dialogue to indicate that you've been skipped ahead in the questline. Ideally, you also want to have dialogue to indicate feelings of betrayal, resentment, and what have you if the Sanctuary group knows that their "saviour" has become a raider since they saw you last. Now, that's not impossible, but since the SS team doesn't exactly have access to the original voice actors, it means either hiring a convincing impersonator (no guarantee you can get a close enough imitation) or, what the SS team and other modders usually do when they need an existing character to talk, you recycle Vanilla dialogue. That can be harder than you'd think. You're stuck with whatever inflection the original VAs used, splicing things isn't always possible, and sometimes you basically have to take a hammer to the flow of conversation and beat it into a workable shape.

Don't get me wrong, I think that the end result would be great, from an "attention to detail" standpoint. Sanctuary getting built up into a city under Preston's control makes a lot of sense. I'm just pointing out that it would take a fair bit of work to do it in more than just a superficial way.

I hadn't thought about the dialogue side of things, but I've had a look at what Bethesda recorded, and there's a lot of good stuff that would fit perfectly. There are lists available of all the dialogue that exists in the game files, such as these ones:
https://fallout.gamepedia.com/Sturges'_dialogue
https://fallout.gamepedia.com/Jun_Long's_dialogue
https://fallout.gamepedia.com/Marcy_Long's_dialogue
I've been poking around in the game files, and most, if not all, of theses lines were recorded and put into the game files. Combined with Preston's negative affinity conversations, it should be possible to portray their resentment towards the player. It's almost as if Bethesda had originally planned to let players side against the Minutemen/Quincy Survivors.

I'm not sure if I see the same lines you do, but it's also important to hear them. Sometimes the way they're spoken really changes the meaning.