With all the new sites up and running it gets more and more difficult for users to really know the scope of a site and thus a lot of questions a migrated to places where they are (probably even more) off-topic, see e.g. here. So I request the opposite of Allow migrating if you have enough rep on either the source or target sites:
Allow migration if you have enough rep on both the source and the target site
(the target reputation required could be a bit lower, say 5001)
That way questions could only be migrated by users familiar with the scope of both sites who can really estimate where the question is better. Maybe those who don't have the rep on the target site could still be able to cast a flagging-only-but-not-counting-as-vote vote...
1) Courtesy of Tim Post's answer