In some way it is already implemented, because you can only build an army in the city where your military academy is.
Yes and no. The concept is similar, but UIs are more about requirements whereas the Military Academy is a hard-coded feature that applies only to units with the 'Army' characteristic. In fact, just adding a 'Requires' caption in the Unit window would seem be simpler than messing around with the Military Academy function --which I am not fond of, by the way (having to build a small wonder just to build Armies seems wrong to me --but that's just my opinion).
The only problem is, that you would have to make the AI smarter to get after those improvements and watches his own.
Let's forget the AI for the moment, shall we? And if the AI is to be a factor in the discussion, all I can say is that it presently can't handle other aspects of the game and people have accepted that, so it shouldn't be a problem now --naturally, this is not as much of a problem in Multiplayer (although I'm a Singleplayer gamer myself).
Whilst they're about such tweaking, maybe they can also have improvements that require a specific resource 'Within City Radius' (like the Iron Works Small Wonder), and Obsolescence for Small Wonders AND improvements-that would be sooo sweet !!!
Some City Improvments like the Nuclear Plant require Uranium but the reason why most don't is probably because it would give those with the required resources too much of an advantage. That said, it's a good idea and just needs more improvements (there are too few of them anyway --including new abilites) to offer an alternative to the already existing improvements; i.e. building a Diesil Plant that requires Oil instead of a Coal Plant that requires Coal (the Coal Plant small wonder should be a regual improvement).
In applying this to Improvements required to buid Units, I would suggest that if such improvements were dependent on a certain Strategic Resource (e.g. War Factory requires Iron), the units requiring them would not be buildable --of course this would be somewhat redundant as units already require resources to be built, but I'm sure you can think of a better way of applying UIs to Strategic Resources.
As for obsolecance of City Improvements and Small Wonders, it would just be a matter of making the abilities of City Improvements, Small Wonders and Wonders applicable to all three (i.e. the Wonders 'Obsolete' also available to improvements and SWs). That way, you could ensure that once a UI is obsolete, the unit can no longer be built no matter what (presently, units can still be built even when they are obsolete as long as more advanced cannot be built due to lack of that resource).
I would think that making this change would be little trouble for designers, and God knows people have asked for it enough --it really should have its own thread --unless it already does.
I think if I were to summerize what types of units should require improvements I would say that all mechanized units fall under this catagory.
For instance, Tanks/Mech. Inf. require War Factory (or an already existing Improvement like Manufacturing Plant), Fighters, Bombers, ect. require Hanger (or Manufacturing Plant + Airport), Battleships, Carriers, Subs require Ship Yard.
Lighter as well as more primitive units would not require UIs (like it is now with all units) as they aren't sophisticated enough to require special manufacturing facilities.
This also serves a greater purpose in terms of gameplay: in the early game most civs have low production potential, thus UIs would be too wasteful and would limit the production of units too much, but in the later game the need for facilities (UIs) limits production of heavy units as production potential is far greater (thus can more than handle the economic cost and you don't have cities all over the place and of all sizes building heavy units --limited only to cities with high production where it's worth the expense of building and maintaining UIs).
Clearly things like initial costs in gold when an Improvement is built and being able to Divert Production (see thread) are factors that could further enhance the strategic element of UIs --aside form being good (in my opinion) additions to the game, with or without UIs.
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