Ironically the 3 I would have most wanted were all approached but it just wasn't meant to be;
Whitney, Clive Davis didn't want her overexposed because of her forthcoming album at the time.
Prince saying they didn't need him on Bad, I just wish that they had attempted it and a demo existed, would have been fascinating to have them both on a track and a major selling point to a Bad Anniversay release.
Madonna, In The Closet turned out phenomenal with Princess Stephanie; her spoken voice is rather arousing (admit it!!) and Naomi Campbell is great in the video, perhaps again I would just like a version with both in existence. The Dangerous album was the perfect time for a Modonna collab.
Elton and George Michael would be my other choices and HIStory would have been a perfect time for either, especially George Michael, maybe a duet on Why, that would have been massive imo.