Re: Which mechanic to increase the late-game difficulty do you prefer in Total War games?
What I personally value the most in the late-game challenges is their plausibility and not how hard they are to be overcome. Attila's solution is a good example. The Huns are really a pain, with their full armies getting instantly resurrected, unless their faction leader is killed five times, but it's a very annoying experience. Besides defying natural law, it is boring, unrewarding and encourages the player to take strategically unsound decisions: You must maul the enemy forces, but not wipe them out or otherwise they will get instantly replaced by a fresh and intact army.
Overall, I am not completely satisfied with any approach, but I think Rome I had taken the correct path, but I don't mean the civil war, which was after all available only for the Roman factions. In my opinion, instead of resorting to the cheap and easy solution of throwing uncountable armies to the player (whose defeat is not even that difficult, since the implementation of the automatic and free replenishment), Creative Assembly should have tried to simulate the conditions that actually prevented huge empires from conquering the entire world. Did the Romans fail to conquer Germany, because the tribes had an infinite pool of manpower? No, ancient Germany was a poor province that couldn't generate large revenues and the lines of communication with the imperial center were too long and fragile to maintain a constant military presence.
And here comes the "Distance from Capital" mechanic: The more distance there was between your capital and your province, the larger and the lower were corruption and public order respectively. This could lead to expansion actually negatively affecting your treasury. I always prioritise the economy, but I have often witnessed my income declining, while I launch large expeditions in isolated territories, like Texas as the Mayans, but also even the Balkans as the Persians. I didn't go bankrupt, because I had already deposited way too much money, but the problem lies with the economy (wealth is basically infinite and since Empire it increases exponentially) and not with the feature itself. I wish CA would invest more in that feature, instead of scrapping it completely. Having to deal with rebellions and budget cuts to secure your vast empire's frontier sounds to me more interesting and historically authentic than exterminating horde after horde.