On the tour of Chem SE I saw a guideline about not asking "Non-mainstream chemistry" questions. What exactly would non-mainstream questions be?
As Potter Stewart once said "[We] know it when [we] see it."
I don't think we have any set criteria for this, but, in broad strokes, we're talking about speculative things like "What would Martian nucleotides look like?" or "Can I create [X] using alchemy?" I know the latter has some historical significance for chemistry, so questions regarding how that influenced modern methods would be okay.
Those are a couple of examples that I can come up with offhand. Really we mean anything that is overly-speculative, rooted in fiction, or that has largely been long disproven by fact doesn't really belong an a site that is seeking to be a repository for solid chemistry knowledge.
While we don't have a fixed policy for it yet (if there are cases where the community is undecided on the mainstreamness, we can come up with one), the relevant policy on Physics.SE should give a good approximation to what we define as non mainstream.