Switch the situations of the Roman and Chinese Empires. The Roman Empire becomes a stable, fairly homegenous empire, while the Chinese Empire is broken up into a heterogenous collection of nation states. We'll say the POD should be after 31 BC, when Augustus becomes the first emperor. And no climactic changes, those are way to easy.
My best shot is to have the barbarians for some reason decide to do an about face and go harass China instead of Rome, but I really don't see why they would.
My best shot is to have the barbarians for some reason decide to do an about face and go harass China instead of Rome, but I really don't see why they would.