Jubong
I accidentally deleted my comment, I'll send it again With a colonial power like Spain France or England during the 1500s/1550s With the Byzantines around 1450 turning history around and taking back lost territories to form the Roman Empire again Unify Russia with muscovy
I agree the wars in Europe are more interesting, but about the period of the Byzantines between its end by the Ottomans and the time of the Empire of Nicea, this choice varies from the points that you prefer with Nicea you would have a much larger initial territory and more initial enemies easy ones having a more medieval vibe without the cannons and muskets. Already with Byzantium you would have only Constantinople with a fragmented army and a very powerful initial enemy at the beginning, at that time you could have a greater development with firearms and if you want to even open a colony, thus being a period with more diversity. Now what to choose depends on your taste and the challenge you want as a writer.
Jubong:Hmm... Colonial powers are intriguing but anti-climactic with all the sicknesses and such... But battles in Europe at that time were pretty much interesting but mostly involved ottoman expansion. Byzantine would be great though what do you think about it taking place during the fourth crusade?