According to a Britney insider that spoke with X17, Britney and Justin Timberlake were in fact supposed to record a song together. Justin & Timbaland wrote a song for Britney, but was never recorded due to scheduling conflicts. The insider says that Britney was prevented from going to Europe to record with Justin due to the: "stress of everything going on here [in LA]."
While yes, I do think that the track could have been bad ass --- I feel that if it were a huge success, that people would give Justin the credit of "Bringing Britney Back" instead of Britney. Guess we'll never know.