Since there's so many ideas floating around, I think it might help to discuss options in terms of advantages and disadvantages.
This is a good idea.
I think my preferences are summarized by
http://forums.civfanatics.com/showpost.php?p=10096441&postcount=826
I think its possible to avoid the "all-or-nothing" approach if oil is sufficiently common but scattered and low-yield (2 per field).
The whole point of Civ5 strategic resources is supposed to be to get away from "all-or-nothing", which is what we had in Civ4 where a single oil well could support unlimited tanks.
I think the basic argument is: strategic resources should matter, but there isn't enough room to make more than 2 late-game resources be important. If you have aluminium but just make it for high-end aircraft (stealth bomber, jet fighter) then thats a really boring and uninteresting resource.
I think there is possible already excess late-game units - how often to you use or build AA guns and their upgrade, and AT guns and their upgrade? So I don't think adding more units is the right fix.
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Now that I think about it, that actually makes a lot of sense... CKN have -2 strength and Logistics, so Longbows could start with -2 strength and +1 range.
Well honestly I think 14 strength is too high for crossbows; ranged attacks need to be well below the strength of the best units of the era.
I'm not sure crossbows should 1-shot pikemen. Compare spearmen:archers vs pikemen:crossbows.
But the problem is most severe on the longbowman, so fixing this at least would be a good compromise. I also don't see why longbows should give England 3 move infantry for the rest of the game.
A ranged defense penalty seems reasonable for Hoplites, or what about a reduced bonus vs horses?
I think a ranged penalty makes more realistic sense [the big difference with hoplites is that they're in tightly-packed slow-moving formation), and it gives them a vulnerability that anyone could potentially exploit, and it means that they'll take more damage from cities and so will still be noticeably worse city attackers than swordsmen will.
If you're making spearmen strength 8 and hoplites strength 10, a 20% ranged attack penalty would make hoplites on par with spearmen vs ranged attacks.
I'd be fine with reducing their bonus vs horses too though, so they retain the same anti-cav abilities as spearmen but are superior in melee combat vs anything else.
It's difficult to balance a unit with +2 strength, 5 moves and faster great general creation
One possibility that is a bit more out-there; to emphasize the fact that these were awesome, but were historically a Praetorian-guard type unit (small group of elites), make them extremely powerful, but have them consume 2 horse resources, so you can only build 1 per horse tile.
Understood on coal, dummy resource seems like the best "fudge" fix.
What makes it particularly frustrating atm is that it blocks upgrade paths. Caravel and Frigate upgrade to ironclad, so I can't upgrade them to destroyer when I've already used all my coal on building factories.