Playing Three Kingdoms, I have just noticed something that bothers me about the recent Total War titles (sometimes it takes a bit...):
I'm playing my first campaign at the moment and have stumbled into a few battles that I couldn't win because my units were simply outdated. Now, that is of course fine, but I noticed the other army consisted almost exclusively of elite units (I forget the name, but they are all unlocked at the ends of the reforms-tree).
Somehow that began to bother me. I updated all my armies to consist of almost exclusively these expensive units. I then remembered how this was handled in Empire and Napoleon (and I think was dropped since then, but I may be wrong): elite units were limited to a certain number - you couldn't have more than, say, 4 of them.
I understand the impulse to not limit the player in enjoying late-game armies and adding in some variety. You don't end up using the same units from beginning to end but upgrade them continually. But it also broke my immersion somewhat and I think it limits the excitement of having elite units when they are unlimited.
I remember in Napoleon I thought carefully about how I should distribute the limited elite units in my armies - they were actually very valuable. In Three Kingdoms, last night, I felt like they are just the next level of units that I will now have to use almost exclusively - no need to mix it up much. ToB handled it a bit better too, I think, because through the recruitment system you couldn't just spam the higher tier units that easily and speeding up their availability came at a considerable and noticeable economic cost (primarily food). I feel like there is no such limit in Three Kingdoms. At least the Records mode should put some limitation on here to make the units feel unique and valuable.