My idea of a perfect followup to Empire Total War would be a game that subsequently carries on the timeline to include up to the early 1900's. Field battles are relatively comparable to the earlier periods depicted in ETW, just with increasingly deadly weaponry. Plus a greater importance on looser formations in the later periods. Whether as a massive sequal/expansion which could be intergrated with ETW, or as its own standalone game (Going from 1700-1900 would be amazing though. Perhaps one day we can have 200bc-1900 in a Total War game :D)
There's all the fun of the great colonial expansions, great battles and wars to be re-enacted, Britians explosion as a global empire, unification of Germany, Napoleonic war, colonization of Australia and dividing up Africa, Waterloo, Crimean War, the Zulu War, the American Civil Warm, the 'Ole West, fun for the whole family.
You have all that great pomp and ceremony, the Brits will have their redcoats and the Germans their Pickelhaube. Advancments in weaponary and research in the final periods will be the key to winning battles, culminating in the final developments of Breech loading rifles, Needle Rifles, Martini-Henry's, Smokeless powder and in the last few years the bolt action rifle and Machine gun.
Naval battles will see the Ironclads and pre-dreadnoughts duking it out with increasingly large calibre guns. Ramships and torpedo moniters.
It wasn't until the First World War that the great industrial nations went to war, and wars escalated from Total War scale battles to the millions. So prior to that it is totally possible on the ETW Engine. It would be massive epic and totally fitting the Total War series.