I recently marked a question for migration (a week ago) and when it asked me which site to migrate it to the only option was the World-building Meta. Is this problem currently being addressed? How long until it should be totally resolved?
Generally speaking, specific migration paths are established after there is a proven sufficient need for questions to be migrated to particular sites. We have not yet reached that point on Worldbuilding.
Note that you can always "flag" a question for moderator attention and suggest migration to a particular site, and one of us will look into it and take appropriate action; a diamond moderator on a site can migrate any question to any site in the network, regardless of established migration paths. Do keep in mind though that generally speaking, if there is site scope overlap such that the question is appropriate on more than one site, then the asker of the question gets to decide where the question should go. Hence if the question fits here there's usually little reason to request migration elsewhere just because the question also fits elsewhere.