In a related post I've asked (again) about custom 'off-topic' closure texts. As I note there, the second thing we need to address if closing as off-topic is a reasonable set of places on the StackExchange network that questions might be migrated to.
Moderators can migrate questions to any other network site, but for normal users only predefined migration paths are available. So what we need to do is decide what those paths should be. I'm not sure how many we are allowed, but I think up to say three 'common' cases would be a reasonable request to the Powers.
What I think I'm after here is one migration path per answer, with a reason why it would be useful (say one or two examples).