Honestly, I think even a 'Muhammad is a peaceful Christian and there is no sudden Arab expansion in the 7th century' scenario could be interesting. That at least would probably mean the continued survival of the full-strength ERE with its Levantine & Egyptian borders, the Sassanid Persians (I've read that Zoroastrianism was losing favor by the time Islam came knocking, some Sassanid princes like
Anoshazad had become very sympathetic to Christians and that Nestorianism had briefly become the largest religion in Persia, so...Christian Iran & Central Asia in the long term, perhaps?), and the balance of power between them.
That said, Muhammad being some sort of early Arab Luther/Calvin could work too. IIRC Christian iconoclasm first became a big thing in response to the Muslims' battlefield successes against the Byzantines, but the seeds were sown as early as the huge spread of religious icons under Justinian. Some sort of iconoclastic, but still clearly Christian, sect replacing Islam as the religion of the Arabs if they still go on their conquering spree would make for a very different Arabic empire indeed, though other than Ebionism (adoptionist Judaizers) I'm not sure what other doctrines they might follow or come up with outside of the already-established Miaphysite and Nestorian ones.