The list of migration options when closing as off-topic should be dynamically generated, based upon how much reputation the user has on the destination site. If they have none, then the only option would be to the child meta.
Currently the migration list is a fixed set of Stack sites where the question can be moved. Migration to other sites is possible only by voting to close (or flagging into the review queues).
This is OK and it makes sense.
However, if the user wants to migrate a question to another site already and has some reputation level (e.g. 300) on the target site (e.g. unix.stackexchange.com), this site should an option on the list.
This will guarantee that user has knowledge of the target site's topic and/or scope.
The other current issue is that right now the list is static, even if the question wouldn't belong on any of the options.
Anyone who reaches vote to close level can vote to move without any knowledge to a set of sites.
Proposed - Not enough reputation on any target sites
As you can see, user still may mark question as off-topic, but can not vote to move. Also important thing is link to reputation requirements.
And - reputation requirements reached on two sites
The per site meta should be always there, but if the question is technically specific, again, can end up in the wrong place.