All these barbarian infantry and tanks...
It would be nice if barbarian ceased to spawn at some point, let's say Renaissance, and instead of them we could get religious zealots (spawn in fog of war influenced by a religion), rebels (spawn because of low happiness), pirates (coastal bases, 1/2 time for spawning ships) and revolutionaries (spawn as a result of revolutionary wave). All such faction could spawn several units at once and be a real threat like in Civ 1. On deity in Civ 1 rebels often had bigger army than my own, even if they were a little behind in tech.