If a question is about a passage which can be clearly identified, and which is in just one part of the Bible, I think you're right that we should edit the question to explicitly mention it.
But I don't think we should necessarily do that for the Gospels. What you have written above implies that for a passage like the transfiguration each Gospel is interchangeable. Or perhaps instead that we can simply harmonise the passages, and that a Gospel harmony is as relevant as the Gospel texts themselves.
While Gospel harmonies have their place, to say that we should interpret any question about the Gospels that doesn't explicitly state a passage reference as instead referring to the relevant part of a Gospel harmony is something I cannot do. The Gospels are not interchangeable, and any Gospel harmony is a work of human analysis, not a primary text (and especially not an inspired text, for those of us who believe in divine inspiration.) I don't think we can actually do Biblical exegesis on a Gospel harmony. We could study one, but the result would not be an interpretation of the Bible.
Alternatively, such a question asks us to consider the texts separately and bring them together - which is theological synthesis, or systematic theology - another important discipline, but not the topic of this site.
So for these reasons I think the question should stay closed, until the author specifies a particular Gospel they want answers to investigate. Maybe there wouldn't be any practical difference, but I think it upholds the important principle that the subject matter of this site is the investigation of Biblical texts, and not theology or Biblical topics more broadly.