I very much doubt this would ever happen, but I would be here for it. I'm not the biggest fan of Christina's live vocals, but I'm always slayed by the recordings. I'd be interested to see what they would come up with.
I actually think Prima Donna, Glam, or I Hate Boys from Bionic would have been perfect collaborations with Bey. (Glam + Rih/Gaga/Bey would have been BEAST. )
Beyonce went on TRL and said that Xtina and Justin timberlake were artist that she liked and she hoped to be an artist like them or something ( for got word for word so i'm paraphrasing)