This is a purely hypothetical situation based on a possible real-life event.
The Roman Expansion East was hampered by the Parthian Empire, notably at Carrhae, and the Chinese Expansion West was too hindered by the Kushan Empire and these two Empires kept these two superpowers from directly colliding with each other. There is a hypothesis that around 36 BC, some Roman prisoners captured by the Parthians were displaced to the east to an area that was taken over by the Xiongnu and hired as mercenaries, and at the battle of Zhizhi there is a Chinese account of seeing a 'fish-scale formation' advancing towards the Chinese. These were too taken prisoner and were finally settled in a place called Liqian (pronounced like 'Legion').
Hypothesis aside, this made me wonder, what would have happened if these two superpowers collided? The scale of the conflict, the casualties/cost, the political and economic implements involved and ultimately who would be likely to win. Personally my money is on the Romans but it would a huge sludge for both sides but I'm happy to hear other ideas