Well, they certainly would have acted as a counterbalance to Rome; perhaps they even could have stopped the Mongols sweeping across most of the Central Asian portions of the Empire!
I have read books about this topic, and a few experts said
That if china was unified they wouldn't have found a need to innovate thus butterfling away the industrial revolution
Making a world that is radically different
Like it's colonies of the United States of Xi'an or the Union of Yang Socialist Republics wouldn't have been the only two superpowers of today.
Ah...but both Franks and Germanics were pretty much assimilated by the Romans when Justinian's (and his successors') army successfully reconquered the former provinces (unfortunately he didn't reconquer Britannia and now those pesky "English" claiming that regional backwater as their land) and even manage to conquered Germania, as well as built the Great Roman Wall along Vistula-Dniester...
While the "Chinese" people were fragmented and warring each other since 3rd century AD after the collapse of the mighty Han Empire...so maybe you need a ruler with charisma as powerful as Justinian to re-unite the entire China...