It's Prince, who performed a rock version of the former Beatle's classic "The Long & Winding Road" in concert. When McCartney heard a recording of it, he was impressed and expressed the desire to "do something" with the long obscure track. 

"Listening to the recording, McCartney noticed Prince made the song "kind of rocky" with "some really good guitar on it." Hearing another rock icon reinterpret the Beatles classic got the creative wheels turning in Macca's head. “It’s really great, so I’m going to ask them [for permission to use it]. Because I could make it into something really good.”

Paul McCartney Hopes to Record a Posthumous Prince Duet