<insert obligatory comment about Mexico and its relative distances from God and the US>
See, that's what I really don't get.
Given the spirit of Peshawar Lancers was basically "Adventure novel genre on steroids", you know what would be a perfect fit in that scenario?
A surviving USA that's basically the Wild West on steroids - where Yankees, Southerners and Freedmen all have taken turns turning everything from the Rio Grande to the Darien Gap into Texas/California/Arizona 2.0, with Latin American influences akin to the Indian ones seen on the Angrazei Raj... and one where the once the "Southern" frontier closes, now have the Great White North to resettle and reclaim. A USA where the frontier will never fully close, not until the Climate fully resets anyway.