The TW engine as we know it is designed to support a battle system based on lots of prolonged melee with troops in formations, which is why Rome will be the best yet. The formations in Shogun and Medieval are over done from a historical perspective, for the most part. This will not be true for Rome because legions and phalanxes really did march around in combat like the Coldstream Guards on the Queen’s birthday. I doubt samurai or men at arms were so precise as they appear in TW.

Once you reach the 18th century, firepower is so overwhelming that melee becomes less important. For all that is said about the bayonet, wounds with them were a tiny fraction of artillery and gunshot casualties. Charges still happened but usually one side or the other broke before contact or else the “melee” actually became a very short range gun fight. In a Napoleonic battle assaults should see a route almost every time a charge is conducted by either the attacker or defender before much melee occurs. This isn't the game people want to play IMO. Cavalry still charged home but it became very brittle and vulnerable with so many guns about and had to be used with great care. By the mid 19th century shock cavalry was a complete anachronism and troopers became mounted infantry, occasional battle aberrations not withstanding.

For these reasons it is not likely that Total War will go to into the World Wars. Formations are gone, ranges are great and genuine melee rare. Anything up through the 30 Years War might work. I’d like to try my hand with pike and shot in the 17th century.