Earlier this week, Grohl further defined the mystery guest as having "been around a long time" and "very talented -- more so than I ever knew." He also said, "I think I've met this person maybe a few times, but I'd never spent time with this person" but that he'd "be amazed if anyone can really figure it out." Taking those scraps of information as hints, we've come up with a handful of people we think it could be, and made the case for and against their inclusion.
The Case For: Since it's not Adele or Taylor Swift, who else could be called "the biggest pop star in the world" but Beyonce? And, as hard as it is to believe, she has been around for nearly 20 years, with Destiny's Child's first album coming out in 1998.
The Case Against: As a global star and multimedia icon, everybody knows just how talented Beyonce is. And does "Queen Bey" really seem like the type who would want her vocals kept in the background? Plus, she may be upset about that time he joked that "Single Ladies" was "basically a rewrite of the theme from The Jeffersons” on Chelsea Lately.
The Case For: Grohl and Justin Timberlake met in 2003 at the GQ Man of the Year Awards, shortly after the release of Timberlake's solo debut, Justified. "I really like his album," Grohl said of his desire to be introduced to him. "I have that song in my head. He used real drums on the album, which means a lot to me."
The Case Against: Grohl's comment about the singer being more talented than expected could have been true for Timberlake about 15 years ago as he was making his break from NSYNC. But his acclaimed solo career -- and his film work -- has defied critical expectations, so, as with Beyonce, it's hard to believe Grohl didn't know how good he is.
The Case For: Madonna's certainly been around a long time. And despite everything she's achieved, she's still never fully gotten the respect she deserves, with greater emphasis placed on her business savvy, her image and her personal life than the quality of her music.
The Case Against: Grohl and his mother have always enjoyed a famously tight relationship. Earlier this year, Virginia Grohl admitted that her biggest fear of her son going into the music industry was that "Madonna would snatch him up." So we have to imagine that her warnings would still be ringing in his ears.
The Case For: Since Grohl specified that the star was huge around the world, we decided to look outside the U.S. As with Timberlake (and George Michael before them), Robbie Williams successfully escaped his teen-pop beginnings (in this case with Take That) and has shown maturity and versatility on a string of solo LPs over the past 20 years. Although he's never broken through in the U.S. -- 2002's Escapology was his highest-charting record at No. 43 -- his albums regularly reach the Top 10 in many countries.
The Case Against: Even with his global success, it would be a stretch for him to be considered among the biggest stars in the world without the name recognition in America.
The Case For: Given the attention shown to her outlandish costumes and her popularity in the dance and pop worlds, it's possible Lady Gaga may never get respect from many critics. But she's proven time and again that she has a fantastic set of pipes. If she can record with Tony Bennett, why not Foo Fighters?
The Case Against: Grohl specified that he had only met the person in question on a few occasions, but hadn't really spent a lot of time with him or her. The video embedded below of the two of them chugging beer together shows that they clearly know each other. Or maybe Grohl simply can't remember the details of that night.
The Case For: Let's face it, if there's any man whose popularity comes close to Beyonce, Adele or Taylor Swift, it's Justin Bieber.
The Case Against: Grohl specified that the singer has "been around a long time," and, although it seems like longer, Bieber's only been around for eight years. And It's possible that the reason Grohl hadn't been aware of Bieber's talent is because he's really not that talented.