This has nothing to do with Total war Three kingdoms and I understand this might bother someone when this game hasn't been released yet. But I wanted to give this suggestion.
In the future you could make a Total war based on the second half of the century XVI and the first half of the century XVII. This is the era of discoveries, exploration and when the spanish and portuguese empires connected the world for the first time in history. Also, there are differences in the military world before and after this period, that's why I don't add the whole centuries.
As Rome was the symbol of Rome II, in this case it should be the spanish empire, the most powerful nation of this era. There are so many conflicts it totally suits the Total war saga. The spanish and the portuguese empire would have the most advanced navy forces as they were really interested to develop the sailing's technology. Also, I want to see the spanish tercios. One of the best infantry units that ever existed. Comparable to the Roman legions and the Macedonian phalanx, although unfortunately most people don't know about it. It would be a campaign were naval battles are really important. The spanish empire fought Portugal, England, Sweden, German protestant rebels, dutch rebels, France, the Ottoman empire, the italian republics were rivals and of course, all the pirates around the world, european, african and asian. This is a really interesting era, but I hope you don't fall for the lies about the spanish empire, which are many. I guess I can trust your professionalism, but you are british, and I don't know why those are the ones that lie the most about the spanish empire. I'll just say something: The spanish black legend.