All close reasons are going to be significantly changed in the very near future, and we're getting the ability to define our own off-topic subreasons.
There is one extremely obvious close reason that by far outnumbers any other close reason, and that is our requirement to show notability. So this is one off-topic sub reason we should write in any case.
I'm not sure if we need more at the moment, anything else I could come up with applies only to a very small number of questions and probably doesn't deserve a predefined close reason. But if you can think of anything, just mention it here.
What we still need is an exact text for the close reason, just put your suggestions into an answer to this post.