I would really love it if (instead of simply being at peace with the barbarians until you get too strong) the barbarian trait instead gave you the ability to conduct diplomacy with the barbarian state the same as you would any other player. Then you could make a peace treaty after the war begins, but they could also drag you into their wars (or you would at least get some diplo penalties for refusing to help, which would lead to them declaring war on you). Also, you could trade resources with them, agree to right of passage agreements, and even gift cities to them or ask that they liberate some of yours. With multiple barbarian trait players, you could ask that they turn against your barb trait enemies. It would also be nice if you could ask them to focus their attacks on specific cities, the way you can your vassals a,d permanent allies.
Of course, this is probably impossible to code.
If it can be done, could we see these changes in Shadow, please? that and a repeatable AV ritual that makes peace with the barbs (requiring a minimum AC) and breaking all of the barbs old peace treaties (except perhaps with barb trait leaders, so Hyborem wouldn't lose his alliance with the other demons), so that the AV civs must constantly fight to keep stay in the good graces of Agares and the Armageddon heroes, sacrificing each other to do so.