Just a philosophical observation on the OP:
Since God is One ---- by definition, somehow pulling off a one-world religion should not automatically be out of line. But if it is some kind of hodge-podge that ignores real differences, that is another matter.
Here is one not to rule out, and should not be assumed to be `evil' if it does happen:
1) Some kind of reunification of West and East Catholic (Roman vs Orthodox).
2) Some kind, yet unknown, of rapprochement with Islam is just conceivable but not in the 1979-9/11 era phase we are in. And yet there are underpinnings that theoretically would allow it. Its not unlike the very great divide from Protestants, while still having considerable in common.
But a ONE world religion? Nah --- if anything, seem to be moving away from it again. What is generally called the Left seems to be adopting a new kind of quasi atheism-pagan (but the classical pagans would not stand for the lack of boundaries) while all the others are digging in.