...new corporations, new promotions, more balanced things...
Eh, the religion issue kind of ate all discussion. Problem with new corporations is how realistic do you want them?
Couple of corporation ideas...
1 - Blue Star Lines - Requires: Coastal city, Screw Propeller. Benefits: Culture, money. Resources required: Coal, oil, steel. Special building decreases naval build times by 10%.
2 - Frontier Furs - Requires: Rifling, Corporation. Benefits: Culture. Resources: Deer, furs. Special building +1 Happy, -1 Health extra w/ Tannery.
3 - Plate Meats - Requires: Refrigeration, Railroad. Benefits: Food. Resources: Cow, sheep, deer. Special building behaves like follow-on to Butchery with similar bonus.
Promotions -
I had been thinking about
equipment effects on troop quality for a while now. Something like FFH2's bronze/iron/mithril weapons or the wootz steel promotion, but to reflect the fact that equipping a unit
well was expensive in the medieval and earlier period. Case in point, a fine coat of chainmail could be more expensive than a longship to the Norse. This would be a line of promotions, both offensive and defensive, that cost money to implement, available to pre-gunpowder units and becoming obsolete with Matchlock. There's already an in-game precedent for this in the Sentry and Sensors lines of promotions, which only become available when the appropriate detection technologies are there, or the Torpedoes and Napalm promotions for aircraft, which reflect air units with specialized equipment.
Civics -
Similarly to the military equipment promotions, I was considering
doctrinal effects on troop quality. That sounds like a very modern way of thinking, but it works in the Dark Ages, too - a Norse raiding band was much more effective than the Saxon levy it might face, because the raiders were warriors led by a warlord while the levy were farmers. So I thought of a military civic column like this...
Levies (default, drafting possible)
Warlords (Experience bonus, experience gain bonus, troops raised by food, less science) - Military Training (think Sparta as well as Stockholm)
Standing Army (Great General gain bonus, Barracks happiness)
Admiralty (Faster sea production, more experienced naval units, Shipyard/Drydock happiness)
Air Superiority (Faster air production, more experienced air units, Airport happiness)
The balancing issue, honestly, I'm not seeing a lot of the problems other people see, though I believe there should be limits on exploration, as the route by which silks reached Rome was pretty tortuous, not direct trade with China. Limiting the number of resources people can reach will by itself limit resource bonus acquisition, forcing them to colonize, which causes Revolutions to kick in as a civ balancer.
EDIT:
D'oh! Been meaning to do this for a while now; I took Crighton's "Gunships" module, which adds a few new helicopter units, and tweaked it for RoM a while back. Since it's pure-XML in the form I used, it works under current circumstances.
Crighton's Original:
http://forums.civfanatics.com/downloads.php?do=file&id=10573
Mine:
http://forums.civfanatics.com/downloads.php?do=file&id=11469
I think mine is a step behind his; frankly my computer never lasts long enough to build a helicopter force, so I don't notice the loss of the promotions. Anyway, integrating his work should be pretty straightforward based on my experience thus far.