@Kathleen302 Correct, it has to be a completed stay. This is because the rating the guest leaves would not be available to count towards the Superhost criteria.
Trying to get another booking by offering a promotion might be a good idea, but make sure it's not at such a low price that it attracts bad guests who might be problematic and leave a low star review that would tank your rating and also disqualify you.
Superhost really isn't a big deal and you should be careful not to try to acheive it by doing things that would be detrimenntal to you in other ways.