This could be for Civilization V, or possibly a third expansion pack called Civilization IV: A New World:
1. Have all leaders from Previous Civ games to appear in their respective Civ, Split up the Native Americans according to their tribes.
2. Have Graphical events and weather/natural disasters
3. Have the units, civs, Leaders, promotions, and traits from the Thomas' War, Rise of Mankind, MaxRiga, Total Realism, legends of Revolution, XX Century, and the Next Level mods with animate leader heads and have Firaxis design the units, buildings, wonders, LH.
4. Bring back all Wonders, terrain, buildings, improvements, and resources from previous Civ games and the above mods.
5. use Ethnic city, improvement, building (Include's palace), and unit art from Legends of Revolution.
6. Give voices to the Leaders and your leader.
7. Add a few new Civs, Buildings, UB, UU, Leaders, wonders, units, terrain, resources, improvements, promotions, traits, etc. that would not have been mentioned above.
8. Put Blue marble terrain inside the new game/expansion pack.
9. Include scenarios from all the previous Civ games.
10. For hit singles, Hit records, hit movies, give names to them.
11. Include the unit art upgrades from Legends of Revolution. (Like how workers change over different eras) That way, say you have an archer in the medieval era, now you got a medieval era looking archer, but it doesn't look like something from 3500 BC, and the unit gets +1 strength bonus for each era.
12. Include a prehistoric scenario for the Advanced Menu/Mods, where you get cavemen and dinosaur units and have different Civilizations with cavemenish names
13. Have a Movies menu in the Advanced Menu so you can actually watch the movies.
14. When capturing a worker, settler, or a city, the unit and city retain their Ethnic art.
15. Bring back the techs from previous Civs and aforememtioned mods
16. Add a different graphic for the Future Era window, and add Future era units from the aforementioned mods.
17. If the civ is a country (In real world) replace the game's flag for the country's real flag.
18. Actually build cities and make a colony on Alpha Centauri and can change from AC and Earth and vice versa. (you had to control your civ on both planets.)
19. For the Industrial Era+ Make the Flag design look like a Flag and not a banner.
20. Have Futuristic unit art upgrades.
21. Building Cottages takes away from the City Population (Not the big number, Total Population.) That way your cities don't get the "it's too crowded." message all the time
22. Put in more random events, put strange sightings like Bigfoot, Aliens, Loch ness Monsters, Krakens.
23. Include as new buildings: Zoo (Recon Units can capture animals, and place them in zoos which provide +1 happiness for each animal in zoo), Amusement Park, Public Schools, and pet shops, (added bonus if you get the prarie dog, or parrot event.
24. Include Rampaging Animals as a random event.
25. Put in the barbarian animals from the aforementioned mods, and have Firaxis design their own.
26. All Classical units are superior to Ancient Units, All Medieval Units are superior to Classic and Ancient Units, Renaissance superior to Medieval, Ancient, and Classical, and so on. Removes the Swordsman being killed by Warriors, and Tanks being killed by Longbowman.
27. For units battling, have an option where you can have every battle the standard battle, or make it a movie. imagine a GGI movie where your Marines are walking through a forest on a road, and they get ambushed by a malinese Skirmisher. And If the Units are on a Improvement or city, include the improvement or city in the movie.
28. Include the religions, missionaries, temples/monastaries/cathedral, religion wonder from the aforementioned mods, But have Firaxis design them.
29. Have upgrade art for settlers, so they look different through the game.
30. have a unit type for spies, settlers, and workers, likes spies could be Espionage Units, and Workers and Settlers could be Civilian units. Missionaries could be Religious Units.
31. Have a CGI wonder movie when Religion wonders are being built such as the Church of the Nativity.
32. Have upgrade art appear in the civilopedia. For example it shows a cottage in the Ancient ERA, then when the player reaches the Classical ERA the cottage is a Classical era cottage, and so on and so on.
33. Upgrade art for certain buildings.
34. Make a different sound effect for Great Spies instead of the Great Prophet chant.
35. Helicopter units can fly over impassable terrain and oceans.
36. Roads become highways in the Modern Era.
37. Add More Corporations.
38. When there is a unit stack, and there is a Great General in that stack, the Warlord promotion is used on ALL the units in a stack, not just one.
39. Make new LH for Leaders that don't look like their Real-Life counterpart.
40. Give different theme music for leaders so they are all unique.
41. use the units, improvements, buildings, civilizations, and Leaders from Colonization.
42. use the Government types from Legends of Revolution. For Example: American City-State or Malinese Kingdom, depending on what civics you chose.
43. Captured enemy units become slaves.
44. The Mercenary idea that was put in mods like Total Realism.
45. You can secretly make diplomacy and or trades with barbarians. Like what Camikaze said.
46. Modern Palaces are based on What the "Palace" would actually look like. For Example the Palace for the American Civ during the would look like the White House.
47. Add more Eras, which would make the game last longer. Like Old West Era, Whatever Era the 1700s was in, Victorian Era, etc.
48. Give Names to bandit gangs.
49. Put Unrestricted Leaders as an option in the leader Picker screen.
50. Add new Great People such as Great Doctor, Great Mercenary, etc.
51. Add a serial Killer random event.
52. Include a graveyard as a building. It produces +1 war weariness for each unit that dies, but produces +1 happiness for each enemy unit that is killed.
53. Wars are named.
I know that taking the ideas from the aforementioned mods sounds stupid (since it would defeat the purpose of downloading those mods), But Firaxis seemed to have forgotten to add some of the most famous and ancient civilizations that have ever existed, like the Israeli Empire, the Australians, the Canadians, the Polynesians, the Vietnamese, The Mexicans, the Hittites, the Hawaiians, etc. And i hate how Firaxis didn't make COMPLETE ethnic unit/city art. I'm pretty sure a german knight looked completely different than an english knight. And there are a million other ideas they should have used that were made by these modders and I wish that they could be incorporated into the game and not as a mod. Maybe in the forseeable future there will be a way to make these mods into a file, send them to Firaxis to make changes to them (By changes i mean fix the art and stuff for leaders, units, buildings, etc.) Then they could put the file back here, and people could put the file into the game. That way the modders are still able to mod and what not and it would still be a part of the actual game.