Legends of Revolution for IV had "Start as minor civ" as a check box option. That way, everybody kinda is a barb. I liked that.
There have been lots of cool representations in the series. I enjoyed using top hatted diplomats to buy and flip barbs in I. I liked the late game camps of AK-47 guerilla guys in III. I liked the huts and barbs in IV.
Granted, there was more which could be done with them. I recall wishing that they take those AK guys and make them more ideological - Communists or religious zealots for example, who tried to cause grief in neighboring lands or yours, if you didn't share their views. Whether you eradicated these camps, or allowed them to flourish would affect relations with the neighbors, depending on religion/ideology.
When IV came along I was trying to get goodie huts to match their climates ( igloos and grass huts for example ) , plus some teepees and desert tents or yurts as nomadic huts. I got a modder to work on it, but his hardware failed and all was lost.
I was hoping V would allow you to secretly negotiate with barbs for mercenaries, horses and proxy wars and pay them in luxuries such as wine and gems. Oh well.
With the new gameplay of VI, I could see a barb tribe becoming a district of your city, be it agricultural, horse farm, military, commercial, etc. not just a threat.
Anyway, there's lots of room for using them in different ways and eras, whether they're called barbarians, mercenaries, pirates, anarchists, guerillas, smugglers, ... or defense contractors