This idea I got while thinking about how awesome bioshock infinity looked. Bioshock has really good tone and flavor and I've made campaign ideas based on it before.
"You sir! Listen to my words for they may well be your salvation! I preach the word of Saint Gerome Faustus, the greatest scientific mind to ever bless the world with his genius. It was Saint Faustus who discovered the plasmids with which we can channel the glory of our Lord! The holy missionary city Providence was built to spread the message of this holy saint. I have been to the walking city myself and beheld its glory, it follows in my wake so that you too may see the glory of the pious! Believe! And you too shall experience the divine for yourself! The living saint resides in the core of Providence and I beseech you all to visit and witness the truth of my words for yourself!"
Yeah I could have done that better. But yeah I was trying to think of another American ideology to take to the logical extreme which seems to be what Bioshock likes to do, though collectivism isn't really American, and then I realized with the Puritans and Mormons and stuff America has had a lot of fringe religious groups that were popular with a lot of people in its history.
Providence would be a city upon massive treads that roamed the untamed west picking up those who desired respite from the lawless land. The citizens of Providence have created a religion around plasmids (or differently named lego genetics wonder drug) and treat their progenitor as a prophet. Tolerance for the treatments is measured in "genetic piety" and a social system of sorts forms over how much one can splice up without damage to body and mind, logically this mentality leads to problems. The "living saint" is a person with so many plasmids and tonics that she is perpetually levitating and must be held in place, glows with an awe inspiring radiance, and communicates telepathically among other things. Yet despite everything is probably one of the most mentally stable of the residents by the time the PCs would show up. Why the party is there could be any number of reasons, industrial espionage is the one I think of. Mutants and Masterminds would be the system I would use as well though I know others would be serve just as well or better. I think its important to have gaining new powers have an element of danger or decay, I never really liked how the player in the video games shot up with all this stuff that messed up everyone in the city and it had no effects really. The residents of Providence could range from well meaning evangelicals to fire and brimstone zealots.