Maybe Chinese philosophy takes a more monotheistic turn, similar to what happened in the Graeco-Roman world. Granted this would probably be an elite monotheistic religion, since ordinary people aren't going to know much philosophy, but perhaps it could percolate down the social ladder enough for Chinese folk religion to treat the various gods more as aspects or manifestations of the one God. Or you could have an Emperor pull an Akhenaten and try and impose monotheism by fiat, although that would likely meet with a lot of resistance and be difficult to make stick.
Alternatively you could have a monotheistic prophet like Mani or Muhammad arise in China, and his teachings could meet with enough success to become followed by the majority of the population,
Or, maybe some merchant brings some Neoplatonist or Stoic philosophical texts from the West, and the new teachings end up getting combined with Buddhist teachings to create a religion/brand of Buddhism with a broadly Buddhist/Stoic ethical system and a broadly Stoic/Neoplatonist view of the divine.