[Astromancer]

Try this idea...

Historically gunpowder has been discovered multiple times and then lost or ignored. Given the multiple independent discoveries of gunpowder it seems all you need are the three commonplace ingredients and some form of alchemical tradition.

So, I simply suggest that an obscure Ionian Greek discovers gunpowder early in the Hellenistic period. By the time Rome starts to invade Greece the Legions are met by phalanx and artillery. Rome learns quickly, but so do the Gauls and others.

Basically use your favorite style of early modern warfare with Classical Antiquity. Or simply have a cluster of cultures with technological advancement forced by competition. Gauls, Romans, Greeks, Egyptians, all striving against each other and driving technological change.

Note: just because of the tech based change there's no reason to exclude magic if you enjoy it. A godswar driving the tech changes would be grand fun.