I have spent who knows how long thinking about this topic, and only just thought to write it all down here, even if nobody ever sees it.
Total War: Victoria would be, without a doubt, the best place for Total War to go to next. An amazing setting for diplomacy and trade, personalities and characters, technology and warfare, the Victorian age would check all the boxes that TW fans and devs alike crave. Here's why:
The base game would very likely be similar to Empire in a number of ways. The first of these, from the campaign map at least, would be the scope of it. Starting somewhere around 1840, the campaign would be able to encompass the political revolutions in Europe after Napoleon's fall, the Crimean War, German and Italian unification, and numerous famous conflicts such as the Schleswig, Austro-Prussian, and Franco-Prussian wars. Also, adding an Empire-style campaign map with multiple "theatres" like the Americas, Africa, India, or even Asia, would make for immensely dynamic gameplay, and fit the Victorian theme of empire-building. From South America to Japan, Sweden to South Africa, the Victorian era has so many good scenarios around the world that would make this game virtually limitless!
Also, since I know TW LOVES their DLC, this game would have so much potential for that:
-American Civil War DLC, with an expanded map of North America, navigable rivers, and a long exclusive scenario (1 month per turn?) of the American Civil War.
-Latin America DLC, with the South/Central American wars for independence, Mexican-American war, coups and civil wars, and the War of the Pacific.
-Scramble for Africa DLC, with (for the first time ever) the African continent, featuring the Zulu wars and other British colonial wars in South/Central/East Africa, playable African nations, influence system for competing colonial nations, etc.
-Asia DLC, with colonization of India/Sepoy rebellion, French and British in SE Asia, Opium Wars, Taiping Rebellion and warlords in China, First Sino-Japanese War, and ties to Shogun 2 FOTS. Even the possibility for Oceania, like colonization of Australia/NZ
What is great about this era as well is the fact that it lies at the integral turning point of technology. In the early stages, much would be similar to the Napoleonic era as far as weaponry, but the tech/research system of TW would come into play massively here as rifled muskets and artillery, breach-loaders, repeating weapons, explosive shells, steam power, and iron armor (the latter two naval of course) make it into a literal arms race. How exciting would it be to actually have your research make a massive difference to your battle effectiveness in the campaign??? And if the campaign ends before the turn of the century, you avoid the issue of the TW engine moving away from linear battles (an issue which mods like The Great War for Napoleon demonstrates). And regarding the technology, it still avoids getting too overpowered or chaotic, while still impacting substantial tactical change. Even with rifled arms and exploding shells, gatling guns and mitrailleuses, you can still maintain linear battles and balance it to make it fun! And let's not forget how amazing it would be to see some ironclads properly implemented (unlike FOTS).
Here's where, besides the DLC potential, this game would have some appeal for the TW devs (at least the ones who turned TW from a grand strategy game to a RPG). As far as diplomacy goes, what era has more political intrigue, big egos, and large-scale diplomacy than the Victorian era??? Even though I dislike the new approach to RPG-esque character-motivated gameplay, the opportunity to do just that in this setting is very much there. As far as economics goes, this would be the time to return to the Empire/Napoleon style of trade and industry. Trade nodes would place a major importance on naval power, as the larger your empire grows, the more exposed your trade will be to raiding/piracy. When we lost that feature after Napoleon, I was very upset. Similarly, we need to return the external, independent industrial "towns" beyond the walls of the major settlements. The current system works great for archaic periods, where everything was largely self-contained in one little area, but Empire/Napoleon had it right with the small towns dotted around the map providing the industry for each region. I remember when one could actually cripple an enemy economy by sabotaging and raiding those little towns without having to launch a direct attack on the regional capital itself!
Moving past my little rant, there are more amazing features which would be critical to this era. Most important of these, at least to me, is the fort. Bringing back fort battles (and for that matter, deployable forts like in Empire) would add so much dynamic to this era between the rise of fortresses (think Vauban) and the death of the fort (ww1). More than that, I want to see the idea that was introduced in Shogun expanded with the new system incorporated into Rome, Attila, and beyond for mixed battles. Imagine having a coastal fort (or in the case of the American Civil War, a river fort) which you can assault by both land and sea. Imagine having ironclads and coastal batteries dueling it out on one side, while your army advances from the other side to take the fort by land. This could add so much dynamic to ports, bays, rivers, and islands, making it not only fun (again, who doesn't think it would be cool to charge a fort under the covering fire of your fleet) but also challenging, as these historically critical locations suddenly become incredibly dangerous (again, think of the "attrition" done to ships within range of batteries in FOTS).
I doubt anyone will even see this, and even fewer will read through it all, and I especially doubt any of this will reach the eyes of the TW devs, but this is precisely what I want to see in a Total War game. As a devoted, long-time supporter of the TW franchise, I know this series intimately and have thought long and hard about what would and would not work. This is not just a "I want this in the game!" kind of spam post; this is something I have carefully considered for several years. Please, let someone important see this!