You could always have the Manichaens lean further into their Gnostic Christian roots. If Manicheaenism syncretises more with Christianity instead of Zoroastrianism, maybe it could come to be viewed as merely a branch of Christianity. That would get you a (very divergent) Christian Iran and eastern Mediterranean. Furthermore, modern European polytheism has a lot of Manichaen influence, despite some people's unwillingness to admit it. Maybe this more Christian Manicheaenism would be seen as less Persian and therefore have more success in the Roman Empire.
Alternatively, either faith could break on to the steppes, spreading far and wide across Eurasia. Again, much like the OTL Manichaens, who reached as far as Japan in this fashion. This option is difficult to accomplish though, considering the Abrahamics would have to first convert or conquer the Romans or Persians, and neither of those seem at all likely - Armenia isn't going to conquer either of them, and the Arabian desert is hardly a good base for an empire. Maybe some missionaries sneak through on to the steppe? It's a long shot considering that they'd have to compete with polytheists, Manichaens, and Buddhists when they get there, but it would make for an interesting TL.