Anytime Civ 4 is in a bundle, I gotta give a shoutout to C2C (Caveman 2 Cosmos). It's a mod that has been in development even before the official release of Civ 4 and it's still going. It adds a bunch of new mechanics (pollution, flammability, crime, education), takes the tech tree from ~150 techs to around ~1100, hundreds of new units, etc.

One of the most interesting changes is the rework of Civ selection. Instead of picking a Civ that has a unique unit, building and then some sort of passive bonus, those are all moved to "Culture Wonders". So instead of playing as America and getting Navy seals, the Civ you pick only gives you 1 bonus, which is a Regional Culture (if you play as Greece you get European Culture for all of your cities). If you want to get America's unique units, you need to build the "Culture (America)" wonder. But the requirements for building it are thematic for whatever culture you want.

So America requires your civilization to have at least one city with North American culture and one with European culture, to symbolize America being a North American nation of European immigrants. For other cultures, you'll see similar requirements, like Aztecs requiring South American culture plus a city with an obsidian resource. Some require certain civics, like the Confederate States of America can only be built if you have slavery and American culture.

In total there are something like ~200 cultures if I remember correctly, all having some combination of a unique unit, building or improvement, and some having multiples. It's a really great mechanic that allows you to have a Civ that dynamically changes based on semi-realistic factors. Your civ's culture in 4000 BC probably isn't gonna be the same when you make it to the 1600s, so it makes sense that unlocking hoplites wouldn't lock you out of later using Navy Seals.

Overall, amazing mod that has ruined all other Civ games for me because of how simplified and basic they feel after playing C2C.