Firstly, religion is sustained from the fear of death, which is resulted from the negativity incurred from the separation of the world population into religious sets.
I doubt this is the only reason, there are tons of them from an inexplicable need to believe in something greater or more than oneself, experiences one can only interpret as religious and therefore inspiring, a set of values that relies upon something else (aside from this physicality) for meaning and purpose and much, much more.
Would there be a world religion, no religious sets would separate the world population, and so no negativity would produce the fear of death which would sustain that world religion.
I doubt it. I'm no expert on the history or religious trends; but from what I gather, there are more evololving each day (rather than less). I'm an atheist, so this is just my view on what seems to be happening.
Still, it would lead to less conflict I suppose. It just seems the diversity with which people think prohibit there being one idea/one doctrine/one set of religious values that all would agree on.