What's the one question you would like to ask if the answer were guaranteed to be correct?
I'd ask, "Is there really any big plan for humanity's existence?"
'Cause, really, it would be nice to finally have that one definitively answered. Sure, I'd then want to know all of the details, but I'd be comfortable with just a 'Yes' or 'No' right now.
If yes, I think maybe we'd try harder to work together to figure out what it might be and how to get there (or we'd just argue and fight even more) and if no, then at least I'd stop struggling to understand why we (as a species) do the things we do.