What would it be? By clearing up, I mean asking him a solid question about the Touhou universe, like "How old was Myouren when he died?", "What was Sakuya's real name?" and (what I assume to be everyone's favourite) "Is Maribel related to Yukari?"
i.e., in the interview where he stated that Sakuya's current name isn't her real name. Assume that he will give a direct answer, not a cryptic response. Also, Touhou universe only, not stuff like "When will TH13 be released?"