I think my main question with r07 would be how he categorizes his own works, and how he feels about their impact with readers. As much as WTC is about mystery, it’s always seemed to me to be less about that and more about the ideas and feelings discussed with in them. Things like the cycle of abuse, social isolation, and the struggle to accept and face reality all underpin the series, which is part of why I personally love them so much, but I wonder how much of that is merely because of his background and thoughts and how much is personal and so on.
In addition to that, seeing how he feels about Higurashi/Umineko in retrospect would be interesting. Especially with the kerfluffle that happened around parts 7-8 of Umineko, I have to assume there are some things I think he’d wish he did or did not do with both works.
Also, I’d see if I can get him to canonize Dlanor/Erika, because that’s important. The final thing would be an open invitation for him to learn english, because I think he’s one of those creators who I think would genuinely find a large following if he could interact with them easier.