Agreed that there is a lot of interesting discussion in this thread. Some points I agree with are:
1) Reveal Iron with Bronze Working (or even Mining). I do this already in a mod I play, and this is great, because it allows you to prioritize either going for IW and many melee units (if you have Iron around) or skipping that line completely. IW might be only 3rd tech, but notice tat beaker-cost of IW is higher than other 3rd-tier techs so you actually need the same amount of total beakers to research IW and Horseback Riding.
2) Giving Swordsmen and Longswordsmen a promotion against melee units would be a good way to make them useful. That way Pikemen would be good against Mounted units, Mounted units are good against Archery units (because of their mobility), Archery units are good against Melee units, and Melee units are good against Pikemen. There is currently no Promotion giving a bonus against melee, I'll look into whether one can mod such a bonus.
3) Giving Archery units a penalty against cities would make sense. This will make CB rushes less of an option and thereby also make Siege weapons more useful (which suffer at the moment). I currently play with a mod that give all Siege weapons Cover I promotion which also helps a lot in securing their survivability against cities (an otherwise big issue in my book).
4) Some late-game use of Iron would be nice. I've changed the requirement for Ironclads from Coal to Iron myself becase a) I would never waste (ever scarce) Coal on an Ironclad, and b) it is an IRONclad, duh? This goes some way in solving the problem. Iron as prerequisite for Cannons might be bad for balance. Iron as prerequisite for Eifel Tower and CN Tower is a bad idea, because these are one-off buildings - if you have Iron, it's not going to have any impact that you use one on these buildings, whereas if you don't have Iron, ir gives you an unfair disadvantage of being cut off from these wonders (and if you buy one for 30 turns, what's going to happen when that deal runs out?).
I would have liked some sort of more flexible mechanism for Factories, i.e. they could consume either Iron or Coal or Oil Aluminum or a combination of these and they production bonus should be greater the more resources you put into them. I also think Factories should provide +1 production on any adjacent Coal, Iron, Oil and Aluminum. I guess as an alternative one could add some more industrial buildings (there is already a Coalplant icon, and one could add an Ironwork (changing the name of the national wonder to something else)).
I do agree that strategic resources - all of them - are a very find balance to master, because you don't want them to be useless, but having them be the difference between win and loss makes the game too random. Aluminum was a good example of this, but I also think that they showed a good solution for this by adding the Recycling centre. By the same means, if one makes Iron more important, there should be special buildings that produce small amounts of Iron to at least cover most basic needs.
Someone mentioned that you could get Iron from City States, but the current resource-sustaining mechanism makes this an unviable option, because if you lose allience with that CS, you units will no longer work. It would have been better in that regard with a accumulate-and-consume mechanism, i.e. you get a certain amount of Iron each turn that you have access to the mine, and when you build the Swordsman you consume an amount of Iron which is then gone for good. I would have favored that for several reasons, but alas, that's not how it works (I also hate how you can use your Uranium for a nuke, drop the nuke, and then use the same Uranium for a new nuke - that doesn't make sense).