I frequently run across questions that should be migrated to other sites, but right now the only site we have an option for is stackoverflow.com (as well as unix meta).
Most of the time I want to migrate to either serverfault.com or superuser.com, neither of which is available. Now I do see questions such as this and this which ask about adding more migration options, but it seems clear we're not going to get any more.
Usually when I run across these I either mark to close for some other reason and leave a comment for the asker, or I just ignore it altogether.
So my question becomes; what reason should be chosen when marking the question for close if the reason is because it belongs on a site which we don't have a migration option for?
Perhaps we should have an option "belongs on another site, but one not in this list".
The specific example that prompted this inquiry is this question. It has nothing to do with unix and clearly belongs on serverfault.