First of all, I'm sorry to bump this thread, but I sure as hell ain't gonna stay silent after spending several hours (spread over two days) reading through it (including most of the other threads that were linked to).
I had several ideas, but I only had the common sense to bookmark related posts earlier today, so I don't remember most of them... I'll post a couple that I thought were quite good.
(Please note I have not messed around with the C3C editor very much, so I don't absolutely know what has been added or changed since the PTW editor that I was more familiar with, I can only assume what I got via logical deduction.)
One idea is based on this thread:
http://forums.civfanatics.com/showthread.php?s=&threadid=50603
The idea is that other than a few core units, *all* units would stem from several ultra-rare luxury resources. This way, each civ would develop absolutely according to its start position (since every one of these resources would appear only on a few terrain types)... The mod would require some creative use of costom graphics, and alot of imagination, but it could be done, and would be pretty cool. If we also assume that at the start of the game only a few of these resources are usable, and about once or twice an age a couple more would appear, you could have a civ in the ancient age that is centered around, say, spears, and would have an offensive spearman, a javelin-throwing skirmisher, a heavy defensive spearman, and perhaps a spear-tossing siege weapon. (I know, very unoriginal, but you get the idea.) Then, assuming we set reality aside from this mod (not necessary, but possible), later in the ancient age said civ would discover Magic and happen upon a Fire Node, allowing them to possibly make a Flaming Spearman (undeadish burning guy with a pointy stick ^^) and some Fire-Javelin Tossers, and whatever. In the industrial age they could discover something that gives them speedy offensive naval units that also have a small transport capacity, along with a marine-type unit, completely changing their style of play. The mod could either have each resource allow a single set of units, or have several combinations... The options are really infinite. Only problem, is of course, actually making such an insane mod. Any volunteers? =\
=-=-==-==-=-=
Another idea arose after I read the exchange about how cavalry was offensively superior to infantry, but much more expensive, and was used alot less because of this. Such a situation could be emulated by making many, rather basic units, cost no upkeep, while units that saw limited use due to high upkeep costs would still cost upkeep. Upkeep prices for all governments would be dramatically increased, and free unit count would be greatly reduced (a bit less perhapas for feudalism which fits well with getting alot of free knights). Now, you may say that free upkeep for so many units is unrealistic, but I say that if we imagine there are two levels of upkeep, we can say that the lower, basic level, is so low that the government doesn't even have to deal with it and that it can be taken for granted, while the more expensive, powerful units would require the government's management due to the way they drain the economy. This could add another dimension to the game by allowing for cheap, large, slow, not-as-powerful armies to fight against quick, deadly, and expensive armies. (Yet another dimension could be added by making the upkeep units very cheap, shields-wise, but I don't know how the AI would deal with that.)
=-=-==-==-=-=
About the issue of polluted impassable mountains,
I thought that perhaps assuming we go with the setup where all units are wheeled other than Scout, Explorer, and Helicopter (?), you could have a unit that comes in with Ecology, a Decontamination Specialist, that is a worker that can only clear pollution, and has a very high worker strength (around three times that of a normal worker sounds good), and *is not wheeled*. This would make it so to reclaim those polluted mountains (and jungles and whatever other terrain you make impassable) left over from the industrial era, along with those that get polluted recently before and anytime after the advent of Ecology, you would have to create the appropraite unit for such a job. They would not cost pop, so in Modern Times you would be very likely to have most of your workers join cities, and be replaced by Decontam. Specs, since your workers end up doing mainly that during the late stages of the game...
=-=-==-==-=-=
These are the ideas I could currently recall. Thanks for an excellent read, and I have to say, I admire you guys. =D
(And I apologize again for bumping the thread. ><)
But that was so long ago . . . 
