Any reason why "What is to happen?" is not accepted?
They don't mean the same thing, though. "What is to happen" could be asking about the next century. "What is about to happen" is asking about the next few minutes or seconds.
I think it should be accepted. That's what I wrote and it sounds natural to me.
I agree - 'what is to happen' should be accepted
Using sta in the manner of this sentence is a standard way of saying something is just about to happen. It would actually be incorrect translate it as "what is to happen."
"what is to occur" wouldn't be right? (Plausible, if not somewhat awkward)