Can't even think of a way to make a start on this.
If I do though, I'll try it. I love the look of barb units and cities. And I love those mysterious messages "Barbarians discover warrior code" that sometimes pop up after the barbs have taken a city. Naturally enough they always seem to concern military advances (and always anachronistic ones). It would be great to see "Barbarians discover Code of Laws" though. Would they suddenly get civilised and turn into a new civ do you think?
And I find the barbs most satisfactory as trading partners.
I only saw a barb conquest of an AI city taking place in a visible part of the map once - when multiple attacks by crusaders beat a warrior defender. The AI seem so strong against barbs that I suspect all such conquests come about when a city is ridiculously weakly held - or very early when chariots or legions come up against warrior defenders. Usually multiple attacks by barbs against AI units in the open, even against defenders with no terrain advantages, just seem to result in piles of barbarian bodies.
Different story when the b******* attack me.