This could be utilized so much more. Barbarians weren't only around back in the cave man days. They are still around today in new varieties. I.E. Terrorists, Drug Smugglers, Rioters, etc. Instead of having only Warriors/Horsemen popping up, in the later era have unique units only available to barbarians. You could go a step further and have them start up small cities where anyone who builds close to them suffers heavy corruption. You could go another step further and have them create leaders such Osama/Hussein/Malosovich/etc that look unique in animation that constantly disrupt the world. It could trigger all civilizations wanting instant peace no matter how bad the situation to remove the unwanted new civ/terrorist.