Xiongnu/Xianbei can be its own civ independent from the Mongols, like how the Turks are an independent civ from the Ottomans. They should be strong enough to threaten all but the most skilled (youtien) China players.
Pre-600AD China lacks credible foreign threats to contain them. Compare this with say, Greece, Persia, or even India. Rome has plenty of credible foreign threats, but can similarly benefit from the introduction of a Celts/Gaul civ.
Alternatively there can be more abundant Indie/Native/Barb spawns (similar to current representation of Celts/Gaul), but I doubt such half-measures will be enough in the long run.
I know very few grand strategy games where one can play a completely isolated civ (such as Maya) and still face an interesting challenge. Usually the best challenge are military and diplomatic, coming from foreign threats. China may be the one instance where DoC follows the rule instead of being exceptional.