Currently our Off-Topic Migration site is empty (exempt meta site), so this option is not used.
Can we add the common related sites, for example:
So people can vote in order to migrate into these sites?
While I do like this idea, beta sites don't get migration paths. There is special case in which Writers SE has a migration path to ELU SE. Right now, I don't think we could even be one of those special cases, though, because we are still a relatively young site and we need more time to fully figure out our scope.
Moderators can migrate to other sites, though, so if you do see a question that should be on another site, please flag for moderation attention and say that it should be migrated to another site.