One weird thing I discovered during my weapons & armor research:
Prior to the emergence of plate armor in the 1400s, there was surprisingly little innovation in personal gear since the Iron Age. Padded, cuir bouilli, chainmail, "banded", splint, it's all there, if not before. (Studded leather and ring mail never existed FWIW; the former is brigandine and the latter was an artistic embellishment of chainmail.) Then there were a few weapons developed specifically to deal with plate, and of course Renaissance-era equipment because it came afteward. This is a remarkably short list. So if anyone's interested in a tech level that's anywhen between the Iron Age and High Middle Ages, the entries struck from D&D5 core are:
breastplate, half plate, plate mail, morning star, rapier, warhammer
. . . that's basically it. FYI, I can't find evidence of full chain mail (just hauberks) existing prior to ~1000, but I also can't find any technical reason why the Romans couldn't manufacture a coif or chausses, just that manufacturing chainmail was a royal PITA. Arguably you can put half plate back in as a form of lamellar armor. Heavy crossbows and greatswords didn't exist in Europe until late but the Chinese had been using them since forever.
Point is, as far as campaign adaptations go, this is amazingly simple.
P.S. I don't necessarily mean historical fiction. This is more if you want to encourage a particular aesthetic. Not every Iron Age story has to take place within the Roman Empire, for instance.