“A civil war version where you have modern weapons and everyone else has period weapons would be interesting.”
1 2017 platoon of Marines vs. Santa Ana at the Alamo
1 2017 armored corps and 1 fighter wing vs. Nazi Germany.
1 company of 2017 IDF infantry vs. the Roman legions invading Jerusalem.
US vs. USSR in June, 1945 (not anachronistic, but interesting as Hell to game that one)
USS Nimitz vs. invading Jap Zeroes (had to throw that in).
Those, and a whole host of others, would be very entertaining.
I liked the old PC sports games where you could pit an old 1970s team against the current (at that time) 80’s team. Not sure games today allow that but that was the best part of those games. I loved being the 1978 Yankees beating the crap out of the 1955 Washington Senators.