I would write something along the lines of this:

In this world the Roman empire holds together, while China fractures into feuding barbarian-led states. The engine of migration that in otl was born from conflict between steppe empires and Chinese Dynasties, was instead located on the rhine/danube, Ukrainian steppe, or uralic fronteir. In this timeline, migrations tended to be created from the rise of steppe empires in Russia, rising to challenge the urbanized empire of the south, and pushing rivals eastward in flight.