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
With the Byzantines outside these two eras it would be at the time of the separation of the Roman Empire with the various enemies or when the Byzantines are formed after the fall of Rome other than that the other periods with them are not very interesting. Already in Viking times it would be, for example, with a British country to resist invasions and unite the whole island. In some more modern period, there would be the option of the Napoleonic wars, for example with Naples and uniting Italy and then if you want to reform the Romans. Or if you want closer to our time during the late 19th century, for example, it can be with Brazil where you can prevent the end of the monarchy, unite South America, and carry out reverse colonialism by conquering Portugal and even getting involved in the first war muldial and if you want to change your result. Now, if you want something older, it could be one of the kingdoms that fragmented from Alexander's Empire, such as Macedonia
Jubong:Yea with byzantium there are many choices but for some reason I want to write a dark age byzantium the most as that will be closer to a real roman army and people still remaining at the time who have seen the fall of the western roman empire in their life time, the time of justinian... I already wrote in the early cannon age, so I am thinking of either early modern 1550ish or revolutionary 1700s or dark ages like vikings or byzantium or saxon england