We will soon see the arrival of the new close reasons (a change being implemented across the StackExchange network). This will raise a few question: here I want to focus specifically the 'off-topic' closure reason.
The 'off-topic' closure reason is being expanded, meaning that we will be able to have one or more custom sub-reasons for closure. The idea is that these can reflect 'local practice' on each site. The example we are given here is for the cooking site:
Recipe requests are off-topic, although recipe replacements, etc. are allowed
i.e. this might look on-topic to the 'passing' user, but the community there have decided they don't answer such question, which makes it 'off-topic'.
Worth noting here is that 'too localized' is being removed as a close reason. That's because it was meant to really apply to very localized issues, but is used by many sites to mean 'other things'. So it's possible we may want to have some 'off-topic' reasons that would have previously been 'too localized'.
We don't have to have pre-defined reasons, and I understand that you will be able to leave a free-form comment when voting to close as off-topic. However, we may want some boilerplate text for common cases. We also don't have to decide on everything now: reasons can be added/deleted/edited as we go along.
Answers ideally should contain one piece of text plus an explanation of why it's useful. It will be handy to have those explanations available longer-term. Based on voting, the mods can then use the text(s) selected in the new box. (Two mods have to agree on altering such texts: one to edit, one to approve.)