Pirates

I think it would be more interesting if the pirates were a new type of independent people instead of a culture to pick from. They can spawn on the coastal tiles as pirate hideouts and their basic instincts are of course to pillage tiles and raid trade routes. Doing this will increase their ability to spawn more units. You can make deals with them to attack your opponents and leave your stuff alone however, this of course will have the side effect of making them stronger/richer. So the players would have to weigh the pros and cons of either destroying the pirate hideout or enlisting them to help, knowing that there could be a possibility that they can be used against the player.