There's no reason you can't still do that while enabling their use to a wider variety of civs. You'd just make the UU stronger. Not dissimilar from present circumstances, knights are available, but stronger UU knights are available on a limited offering.
From my view, a civ evolving into Mongolia based on something like # of horses available makes sense. Sort of. Not exactly Mongolia specifically, but if you imagine Mongolia as a kind of stand-in for whatever alt-history X turns into after moving onto a steppe region particularly supportive of pastoralism, it does.
It makes similar sense, conceptually, for civics roughly akin to strategic doctrines to emerge that allow unit selections which conceivably counter such evolutions, both from a gameplay perspective and from the immersion, 7-as-dynamic-living-world perspective. The wagon fort is a particularly good example, because its a common response to that stimuli historically.
I don't expect to see that in 7 but I would delight if such concepts are there eventually.
Was not posting what I would WANT to see, but what has been seen in past games and so should really be expected in this one.
In my Perfect 4X Historical Game (WIP for the rest of my Natural Life, I suspect) I would offer each Civ/player a selection of 'Unique' Units, Buildings, Districts, whatever at various points in the game.
Get Horse Resources?
You have a choice of being able to build a light horseman, a horse archer, or an armored horseman - but to get the horse archer, you have to adopt a Pastoral Civ lifestyle.
Got Pastoral neighbors?
Then you get a choice of Fixed Border Walls (Long or Great Wall, Hadrian's Wall, Offa's Dyke, etc)
OR Militarized Border - every Settlement/City near that pastoral border loses X percentage of its Production (including Food) BUT can build Units that start with a Promotion or Bonus (OR in Civ VII terms, automatically produce a new Army Leader every X turns).
Basically, I would love to see a game in which many of the unique aspects of your Civ are acquired during the game, not given to you regardless of the in-game situation of your Civ. Never fight a war, what do you need with a Military/Combat Bonus? - And more to the point, how do you develop such bonuses if you never fight a war?
My idea of a good game design revolves around:
"Ships in Deserts is Camels"
- If all your Civ is in a land-locked desert, you should not even be able to research 'Boating' or "Sailing" or anything like them. You are stuck with Camels whether you like it or not. You want real ships, look for large bodies of water or navigable Rivers.
