A traditional Total War game set during the Second Industrial Age or later would be impossible, due to scale, and the changing face of warfare. It already takes a heavy suspension of disbelief to play an ancient army no larger than 4800 men, and when you start talking about single armies of many hundreds of thousands being shrunk to the size a brigade, things get ridiculous. Even if the armies were shrunk, the battlefields would have to be immense to take into account artillery even from a relatively short 10 miles away. How would the mechanics for air strikes work? There would need to be mechanics for choosing an area and fortifying it. No one has a computer strong enough to do all that yet.

Also, they'd probably whitewash the hell out of the more recent stuff, like removing all the swastikas from German uniforms or some such crap.