The United States didn't become a full-fledged superpower until by the 1920s, so I think they'll put it in the Contemporary era.
Being a superpower is not a condition for inclusion in the roster. Rather, amplitude constantly repeats the word emblematic instead of unique. The idea is to create a world that lives and that also means filling it with connected civs.
That's why I expect there to be duplicate options - when they have reached 120+ included cultures: the Colonial Time & Founding fathers, but especially the "colonization" of the Wild West is just so very much emblematic. Playing "Cowboys and Indians", Gold Rush and the Civil War with all its Southern Antebellum slavery "glory", just so emblematic. And you can't really depict this with transcended English, Franks, Teutons or Norsemen. As the English themselves by the way are in dire need for an early modern implementation around Henry VIII, pirates, theatre and fighting with Scotland. And the Franks by the way need one with Francis II, the religious wars and the Huguenots as well as up to Versailles. They're missing out on Versailles at the moment!
So again, if you want to provide a full historical picture, you need a few more options. The challenge will be the naming of them, since I do agree, we absolutely need the modern era United States! But let's first enjoy the vanilla game
Unfortunately, in most games 12 turns isn't enough time to build, deploy and use a weapon or Unit unless the enemy is conveniently located a few tiles from your capital!
I think that's the biggest challenge for a historical all-encompassing strategy game. Since Humankind has battles that are resolved in around 3 turns during one turn, that simplifies this problem a bit. But in general yes, the solution is to simplify the units very strictly: going directly for the jet fighters and bombers, chosing one contemporary look for both of them and that's it. Or maybe a future game will do wars like humankind does battles: on a separate map that resolves in one turn. Might make you prepared for war better.