The catalyst (but not the only cause) for this question appears to have been one of my close votes, on a question that was obviously off-topic on SO and obviously on-topic for CR. @200_success is of the opinion that the close-vote should be the generic "too broad" (or possibly "primarily opinion-based"). I respectfully disagree.
Really, this would be a lot simpler if we could flag for any migration path directly. Overloading the "moderator-attention - other" box just adds work for everyone involved, so most good-quality migratable questions don't get migrated (the fact that the obvious off-topic close reasons are nonmigrating and you have to click harder for the migration list is also to blame).
The main justification for only showing the top 5 is a self-fulfilling prophecy: if users don't see the path, they won't use it even when it's applicable.
- If you want to warn users to not migrate crap, give them a warning that they should not migrate crap in the migration dialog.
- If you want to warn users that it might be on-topic in both places, warn them that they shouldn't close things only because it's on-topic elsewhere.
- If you want to only allow migrations for users accustomed to the destination site, only allow initial migration votes from users with some reputation on the destination site.
- If you want to provide a per-site "what should be migrated here?" readme, link it from the migration dialog! Many sites have one, but they are hard to find.
And yes, I have read the previous MSO post, Update On-Topic Help with Links to Common SE Sites. And I agree about "let's do something", because the current status quo sucks.