Hi all,
I have few suggestions for Civ4 mods which I think might be worth considering. I am unable to code it myself (lack of time and expertise), but I want to share these, just for inspiration.

1) resources system
Resources system of CivIV is a bit unrealistic. You can trade resources like horses for dozens of years, but if your trading partner decides to stop, you are sudenly horseless. Also, mineral resources can not be depleted. Maybe it would be interesting to divide resources into three categories:
a) spreading resources (horses, pigs, corn...) - if you trade these resources with someone and have none on your territory, there is a slight chance, that a new resource of this type will appear inside your cultural borders.
b) mineral resources (oil, coal, iron...). These are limited and can be depleted. Every year of usage removes one conter from the resource tile and every unit or buliding requiring it removes some too when finished. Each resource has cca four sets of these counters. If you have proper mining or processing enhancing technology (mining, engineering, gunpowder, steam power, electricity or something like this), you are able to access more of this resource. Recycling center reduces depletion of resources by new buildings and units. This can start an interesting competition between civilizations when their resources are close to depletion.
c) other resources (fish, hit musicals...) - no change

2) military organization civics:
Military organization has undergone important changes and paradigm shifts during history and there is enough variability to use these as a category of civics. It should look somehow like this...
a) tribal (no upkeep; no tech required; no special effects; this is a tricky one, in most primitive societies, every man was a potential warrior, but there were no standing armies - which both leads to a different civic. We however need some "default", so let it be this.)
b) military democracy (low upkeep; iron working; free units up to sum of town sizes; extension of the tribal system - every man is a potential fighter, but they may follow a strong leader on their will and are used to military life. Celts, Migrations period, Homeric Greece...)
c) professional army (high upkeep; monarchy; +3 experience and free Combat I promotion; large body of paid military professionals under control of state authority, military career is possible. Rome, asian empires, modern USA)
d) vassal system (medium upkeep; feudalism; can draft in each happy city; each vassal has a duty to establish a local army body if asked, medieval Europe)
e) mercenary army (high upkeep; currency; can hurry units production by money; 30-years war)
f) universal forced conscription (medium upkeep; nationalism /maybe earlier - monotheism?/; free barracks in every city, can sacrifice poulation to hurry military production; every able citizen gets military training, usually also a military rank and can be anytime called to service, usually large absolutistic/totalitarian empires or democracies during critical wartime)
g) civic militia (low upkeep; liberalism; attacked city "drafts" a defender until it stays happy if attacked and there is no one to protect it; a civic/libertarian concept, Switzerland, small states fighting for survival or otherwise well-motivated)

These are some nice ideas, they have been similar suggestions along these lines before, but these are though out better then most.

On the resource depletion idea the player would have to get a screen ware they could see a list of all there resources by type. Within each type they could see each currently accessed occurrence (and unaccessed ones too) with the remaining quantity in each one and a total "Reserves" that have. Each one could be toggled to "Use" / "Conserve" or the category as a whole. Consumed resources are randomly drawn from the locations that are in use.

Great ideas! I realy would like to implement these in component.:goodjob:

Recently, I was thinking, how corporations can be used to simulate various movements, that are not religions (and switching to such one as a "state religion" would look crazy). Some ideas:

Scouting
Requires: Education and Assembly Line (Education requires no explanation; Assembly line reflects industrial revolution, during which traditional socialization ways, free time usage and education needs among youth in cities changed rapidly, and role of nature in education was being considered in paedagogy. I was thinking about Fascism too, but Assembly Line is close and seems to be more appropriate with regards to all aspects of 19th century youth movements except for uniforms :-).)
Founded by: Great general (reflecting Baden-Powell and also allowing this kind of great person to have a corporation of their own)
Wonder Headquarters: Gilwell academy (acts as scout headquarters, in addition +1 :culture: per city where present)
Local headquarters: Scout headquarters (+1 :gp: per city with same owner where present unless Police state is chosen civic; +1 XP for all units built; +1 relations with other civs with Scout headquarters or Gilwell Academy in their capital if in capital)
Executive unit: Scout leader (more changes in graphics required)
Consumes no resources. Not affected by civics affecting corporations.

Freemasonry
Requires: Liberalism and Construction (While roots of freemasonry may be traced deeper, it became an important part of enlightement period. Adding things like Mysticism to requirements is not necessary, because these techs are too early and almost everyone already haves them as they race for Liberalism.)
Founded by: any great person (Engineer, prophet, artist, general, spy, merchant... you can find reasons for every type of great person here...)
Wonder Headquarters: none (founding city receives a normal Grand Lodge)
Local headquarters: Grand Lodge (+0.5 :gold:, :culture: and :espionage: per specialist; +1 :mad: for a civ with Emancipation in Emancipation not active, +1 :mad: if Free Religion not active)
Executive unit: Freemason Master (more changes in graphics required)
Consumes no resources. Not affected by civics affecting corporations. Cannot spread under Theocracy.

Relief fund
Requires: Medicine and Democracy (medicine is self-explaining, I chose Democracy over liberalism because it both emphasizes equality and allows people to work for a common goal from their own decision)
Founded by: Great Prophet (Great Prophet gets his corporation...gives some more usage to Great Prophets in modern era)
Wonder Headquarters: Relief fund headquartes (+0.2 :) per foreign city where present, acts also as a relief fund branch; +1 attitude from all leaders with Open Borders and relief fund branch in a capital)
Local headquarters: Relief fund branch (+0.2 :food: per a growing city with RFB if starving or would be starving without this bonus; +1 :health: ; + 0.25 :gold: per a foreign city with RFB and Mass Media if unhappy; - 0.25 :gold: per foreign unhappy city with RFB if you have Mass Media; - 0.2 :food: per foreign starving city with RFB unless starving or not growing)
Executive unit: Humanitarian Worker
Not affected by civics influencing corporations.

International Crime Cartel
Requires: Nationalism and Guilds (Guilds are self-explaining; Nationalism because organisations like Mafia were often local irredentist movements)
Founded by: Great Merchant or Great Spy
Wonder Headquarters: Crime Cartel Headquarters (+ :gold: given by Local Crime Cartel Branches; +1 :espionage: per Local Crime Cartel Branch)
Local headquarters: Local Crime Cartel Branch (-1 :gold:; -1 :gold: if owner has no Scotland Yard; -1 :gold: if no Security Bureau present; -25 percent spy operation cost in the city for ICC owner; +25 percent chance of discovering enemy spies if in ICC owner's city)
Executive unit: Crime boss (more changes in graphics required)
Not affected by civics affecting corporations.

I've thought about biological resources. It would be easy to make them necessary to build a building that provides the resource. Bonus Horses is necessary to build the building Horse Herd which is necessary to build the building Horse Breeder which gives Horses as a free bonus. I haven't actually tried this, though I know doing it without the intermediate step doesn't work. You cannot make a resource necessary for a building and also produced by it any more than you can make building give itself as a Free Building.

This is a make do with buildings. What would be really nice would be if you could make a resource necessary to build an Improvement. Improvements can be set to do what I call "Bop" ie produce a Bonus on the tile as soon as they are worked by simply adjusting Discover rand from 10000 to 1 in the ImprovementInfo.
<iDiscoverRand>1</iDiscoverRand>. If you had an improvement Horse Pasture that required Horses to do the Build for it, then you could build a horse pasture and Bop the bonus Horses onto it, and thereafter be free of the original source of horses.

Then there are issues of the AI not using it right, which requires actually knowing how to program.

Something i would like to see is a Corp or set of Corps that can convert Gold Coins into Commerce/food/Hammers because frequantly i find that at certain points in the game i have huge extra gold supplies that i cant do anything with so being able to feed the extra gold income back into the empire would be really useful.