Give desktop visitors different pop-ups than mobile visitors.
Give customers different pop-ups depending on their proximity to you.
Give customers pop-ups at different times of the day (like happy hour).
Give customers a different pop-up depending on if it's their first time visiting your site or 100'th.
Give your website visitors a different pop-up depending on how they found your site.
Give your website visitors a special pop-up and website interaction on holidays.