(endgame umineko spoiler) Probably what lead to the creation of the character of Yasu and what made him focus the “heart” of the novel around that.
It was a very interesting theme, but I would never have expected something like that from the author of Higurashi. (that is way more animu in nature and overall less… grounded in reality?)
And yeah, his views on few things like religion, etc would be interesting.
In a personal/serious discussion, I’d ask if he had any spiritual experiences or if he had visited some powerful shrines in Kanto area.
In WTC related questions, I’d ask about his inspirations for Furude Jinja and Oyashiro-sama to find out concretely. I’d also ask about his vision of the meta world and inspiration for the sea of fragments
@epika, those are really good questions! How did he reply to them? 
Main question I would ask is will he ever release “Land of The Golden Witch” in the form he initially planned on.
After he announced both WTC5, Haworthia and a brand new Umineko Episode altogether in a row, I think the only question that remains for us to ask him is “Will you marry me?”
“What happened to higurashi VR?”
I’d probably talk to him all about locked room murders! (Maybe sneak a few of mine into his hands and see what he thinks, haha.) I get the feeling we’re both people who love all of those works by the Detection Club and delight in the logic games. And then, given my vocation, I’d (not subtly) turn the direction towards philosophy and religion.
My questions from AX?
Well it was quite funny about his answer about Erika and it surprised me how fast he reacted to my question regarding KeiRika. Still felt pretty lucky being so close to him!