In the specific case of your example, that question should not be migrated to any other site in the first place. It's just a bad question, and the first rule of migration is that you should not migrate crap.
Simon is yelling four times louder than a jackhammer. If the jackhammer is measured at 100 decibels, how loud is Simon?
If you absolutely think that a question is of good enough quality to merit migration to a site that is not listed in the pop-up, you should flag that question for moderator attention. Explain why and where you think it deserves to be migrated. Moderators have the ability to migrate anywhere within the SE network.
Jeff has said numerous times that we will not be adding more than 5 locations to the "close" pop-up. Not only is presenting too many choices confusing to the user, but it's quite rare that a good physics question is inadvertently asked on Stack Overflow. The common migration paths are all there already.