Each site can have up to 5 migration targets, 1 of which must be that site's own meta. All of Super User's migration targets are currently in use. Gaming.SE used to be an option, but we found that Android.SE was getting more migrations (manually made by moderators) than Gaming was even with Gaming as a default option, so Android was added as a default and Gaming was taken out.
You can always flag with the "other..." reason and ask for a question to be migrated, but remember, if the question is valid on Super User, don't vote to migrate it even if it's also valid elsewhere.