These are confirmed, but I'll list them anyways.
![Celestial](http://img.photobucket.com/albums/v232/korn0818/ATRL_Smilies_All/atrlers/celestial_zps52052823.gif)
Gaga & Cher
Taylor Swift & Justin Bieber (possibly just a cowrite and not a duet)
Predictions:
Nicki Minaj & Taylor Swift
Rihanna & Katy Perry
Some collaboration will happen between these 3 artists: Shania Twain, Bruno Mars & Taylor Swift
Whether it be Shania/Bruno, Bruno/Taylor or Shania/Taylor, I don't know... But one of them will happen.