I'm not entirely happy with the way new military units become available. The SAM Infantry is the main offender here. Why does my empire automatically switch to SAM soldiers when I discover rocketry when nobody, including me, knows Flight? Where's the causality? How can you specialize an army for attacking a type of unit that doesn't even exist yet or that you have never heard of?
The Aztecs had never seen a horse before the conquistadors came. They even assumed it was one creature until the rider dismounted. Would you expect such a civilization to have pikemen standing by with a +100% bonus against mounted units? I think not. The conquistadors were not successful because their horses had a +50 bonus against melee units, but because the Aztecs had not spend any beakers in researching anti-mounted unit technology... because they never needed it!
It's the same with the 'pinch' promotion (+25% against gunpowder units), that is available before the invention of gunpowder.
What if, instead of building a SAM infantry, you would just build a 'blank' gunpowder unit, which upon completion would be assigned a function? At first there would be only one function available, which would turn it into a standard musketman. Later on (with the invention of rifling) you would get a second option available, where the newly build gunpowder unit can be specialized as rifleman. This would prevent the illogical appearance of SAM infantry, because YOU, the player, are now deciding whether or not this new gunpowder unit should pick up a site-to-air missile launcher (with the discovery of both rocketry AND flight) or that you want to to become a marine, or a 'regular' infantry.
The same principle could be applied to other military types, e.g.: You build a pole weapon unit. You need to know Horseback Riding to specialize it as anti-mounted unit. Which should only become available for research once you have actually SEEN horses on the map. If all horses are still under the fog for your civilization, then horseback riding should not be available to you. And your units cannot specialize as horse buster. But with Iron available they could be promoted as anti-melee unit - let's call it the halberdman (a guy with an axe on a pole).
I'd also like the ability to mount different types of foot soldier on horseback. Build a 'blank' horse unit and combine it with an existing unit. A spearman on horseback would become a jousting knight (bonus against mounted units), because he could bump a regular knight (horse + swordsman) off his horse. A rifleman on horseback become a cavalry. A musketman on horseback would become a weaker form of early cavalry. Archer + horse = horse archer, etc. And I also want to mount a SAM infantry on a horse. I have no idea what you would call it, but why not? The battlefield might actually require highly mobile anti-air units! Also, this:
SAM Infantry on motor bike
Above: Civ 5 if I was in charge.