And the minutemen here would instead be some sort of original medieval type kingdom that emerged on the surface but was trapped in a loosing war against the institute, and were so exhausted by the war that they collapsed into civil war and those are the settlements you'd have to conquer and the meat of the game would be the moral dilemma of picking different factions to become your "vassal" for each settlement to produce different things in a system that actually mattered

Adding to the synth idea, what if instead of being robots at the core, they leaned more into being FEV creations? And that when they rebelled they would originally just melt away, but then the founder of this better railroad, lets call him spartacus, discovered how to postpone this process for ten years, but then spartacus died whilst searching for a cure for their melting, and you, the player, given you're technically their genetic basis, are discovered by a faction within the railroad that worships you as their god.

So there's like a "perfect" but extremely lame ending where you become a vassal of the brotherhood of steel, the minutemen get absorbed into this synth caste system and everyone gangs up on the institute, another where you assasinate your way into secretly controlling the BOS and making them leave so you can conquer the institute and turn it into synth hyperborea ruling over the minutemen as tributaries, a synth murder spree ending where you assasinate your way into couping the institute and create a synth army to take over the world, a minitemen purist ending where you make the BOS and minutemen collaborate to make some sort of permanent inquisition to hunt down synths and mutants, and a worst ending where you reform the minutemen to carry out a mao type cultural revolution, purge the synth cult, use it to take over the institute and create a secret synth organization that rigs the pridwen to explode