Probably the Han dynasty in some roundabout way. Assume the Han expand slightly further westward without infringing upon the Kushanshahs and officially declare certain nomadic and possibly Germanic states in the Pontic steppe to be its western representatives and tributaries. These states then conquer Rome and the Han officially claim these as still vassals and representatives in some roundabout way.
The Han dynasty fell in the early 3rd century IOTL, and I don't think it would be plausible for Rome to fall to the barbarians before this date. Though I guess you could somehow have the Han last into the fifth or sixth century ITTL.