But I'd love to start a Civ like Turn-based strategy game in the aftermath of nuclear war, and slowly rebuilding a town into a nation, scavenging resources, finding lost tech, trying to balance between different factions, etc. etc. Fallout would be perfect for that.
...Spore is the best starting point of mechanics I can think of. Because it has mechanics for going from single person, to small tribes, to cities, to space, with custom-built things all the way up and different features unlocking at each stage.
Going off of this basis, you'd start as a small group of scavenger-nomads, taking stuff from other nomads, fighting raiders, raiding settlements yourself and so on, being able to make your own weapon and armor designs based on designs you have or what specific people in the group know, with various degrees of customizability based on these things.
Next, you make a Settlement, a permanent place to farm, set up water purifiers, keep stuff relatively defended and so on. New features unlocked would be designing structures and turrets, with you keeping the previous designing of weapons and armor for your people. Generators would be a part to attach to any structure, including turret-covered monsters.
From there, you get to Nation, having multiple Settlements/Cities to keep defended, but also having much more in terms of resources to do it with. Now, you can make new designs for robots and vehicles, provided you have people with the right skills, at this point tracked by statistical breakdowns rather than individuals, and infantry are similarly tracked as statistics.
The win-condition would be controlling the entire continent of North America, with the challenge factor for Nations who can field moving cities being that Mexico didn't fall nearly as far as America, so they re-developed into a largely isolationist superpower by current standards. Of course, they are far from united, so while one branch of your Nation is expanding through the anarchy-dominated ruins of the U.S., stomping over everyone outside the Northwest Brotherhood of Steel and the NCR, possibly a few factions in the middle of the U.S., with the Legion either being easily destroyed cannon fodder or going for a Mad Max version of the Roman army for vehicles and robots. Mechanical replicas of mythological creatures? Sure, why not? Chariots driven by steam engines or nuclear reactors inside robotic horses? Makes sense for a later-stage Legion that's started to run into issues that need tech to solve.
...y'know, the Legion developing to adopt Roman aesthetics tempered by Mad Max for high-tech things would actually be great. It lets them be able to be a threat to a teching-up NCR who can field power armored squads without compromising their identity.