it will be a a hyper-militarized modern city time travel-blended inside a ruined, nazi-occupied france, or maybe for added timey wimeyness an alternate reality of a nazi-occupied america. it will deconstruct american militarism by demonstrating the similarities between our modern military and that of the 1940s, and the nazis themselves. there will be sections that function normally, sections disrupted by being transported next to and into old dilapidated war-torn parts of the old city, and parts where everyone is fighting everyone because no one knows WTF is going on with all these nazis and s**t where the coffee shop used to be. and of course this reality has some unethical nazi scientist who creates all the anachronistic technology and magic powers from the modern stuff teleported in. you start off a citizen of the old city, and the modern americans are your allies at first "liberating" you, but we all know how that goes....
just need some way to marry the anachronistic tech. obviously modern soldiers will mop the floor with nazis, so we gotta find some explanation for the equivalent to plasmids/vigors the nazi scientist comes up with. maybe commies can be involved? obviously a very rough idea, but i think it has potential.