With all those rap / sung collaborations submitted to Pop Duo / Group they basically gave away Best Rap Sung Collaboration to Holy Grail
Meanwhile Pop Duo / Group is so competitive that anyone could be snubbed and there's only 5 spots:
Best Pop Duo / Group Performance:
P!nk feat. Nate Ruess - Just Give Me A Reason
Rihanna feat. Mikky Ekko - Stay
Robin Thicke feat. T.I & Pharrell - Blurred Lines
Daft Punk feat. Pharrell Williams - Get Lucky
Justin Timberlake feat. Jay-Z - Suit & Tie
Other possible choices:
Maroon 5 - One More Night
Macklemore & Ryan Lewis feat. Ray Dalton - Can't Hold Us
Drake feat. Majid Jordan - Hold On, We're Going Home
Taylor Swift f/ Ed Sheeran - Everything Has Changed
Basically any of these songs could get it.