We have a mod called Allow Duplicate Cultures that allows every player in the game to select the culture they want even if someone else has already selected it. We've tested it in multiplayer games (played to the end of the game). You can enable and disable this option when you create your map at the beginning of a game. We're interested in feedback if anyone wants to try it.

You can download it here (you don't need to join to download): https://gc2021.com/forumdisplay.php?fid=6

How does the ai respond with this? Do they all pick the same cultures every time?

They do not all pick the same culture, but once they've chosen an affinity, they tend to stick with that affinity through the eras, which I think they tend to do even without this mod (in the above pic, only one player was an AI, the other two were human). That said, it isn't always the case that they do this.

This is great. I hope this can eventually lead to games with more that 10 players.