Yesterday, the Foo Fighters broke news of the upcoming Concrete and Gold; pump-fake hiatus aside, their ninth studio album is due out this September, and it figures to be decidedly different beast than the one that came before it.

2014’s Sonic Highways barreled into existence as the first album to come pre-packaged with an HBO special devoted to each of its tracks. Every song on Grohl’s “love letter to American music” was inspired by a different American city, and the band traveled to each locale to be filmed collaborating with its resident rock 'n' roll legends. The Foos have never been big on guest stars, but Sonic Highways pushed them towards the modern pop industry’s feature-happy circus on distinctly Foo-friendly terms -- co-writes with storied, celebrated musicians, most of them somewhere on the rock spectrum: a song in California with the Eagles’ Joe Walsh, a song in Nashville with Zac Brown, you get the idea.

But yesterday's big announce hints at a very different kind of team-up. Speaking with BBC Radio 1’s Annie Mac, Grohl revealed Concrete and Gold will feature numerous guests, including “some that will really surprise you.” He continued, “There’s one who is probably the biggest pop star in the world and -- I’m not kidding -- sings backup on one of the heaviest songs on the record, and we're not telling anybody who it is.”

Biggest pop star in the world, eh, Mr. Grohl? If he isn’t messing with us -- and his definition of the world’s biggest pop star isn’t completely bananas (which, sigh, it could be) -- there’s a limited number of people who actually fit the bill. One big collaborator for the record has been revealed in producer Greg Kurstin, who’s had a hand in many of the most successful (and critically-acclaimed) pop albums of the past decade. His presence could be a big hint towards figuring out the mystery guest, who could be…

Adele: We begin our list of suspects with the most likely one. Greg Kurstin worked closely with Adele on her Grammy-winning 2015 album 25, which included producing and co-writing its biggest hit, “Hello.” Adele and the Foos have never been prominently linked before (unless you count being mutually adored by the Grammys), but we could see an old soul like Adele getting along with the modern day rock ’n' roll torchbearers. Singing backup isn’t something she’s accustomed to, and she hasn’t hopped on someone else’s song since the early stages of her career. Still, we think Kurstin and the Foo Fighters could convince her to buck that trend. Start engraving that Grammy now!

Beyoncé: Out of everyone else with a stake to the "world's biggest pop star" title, Bey's probably the next-most likely to hop on a Foo Fighters track. Sure, Beyoncé and Grohl isn't the easiest pairing to picture, but don't forget about Bey's recent collaborations with Jack White (Lemonade's "Don't Hurt Yourself") and Coldplay (their 2016 single "Hymn For the Weekend"). Teaming up with veteran rockers has been a useful flex towards spreading her massive influence. It does seem, though, that Beyoncé spent most of 2017 with bigger priorities than kicking it with Dave Grohl.

Ed Sheeran: There's no clear connection between these two, other than the fact they're both headlining Glastonbury this week and, well.... Sheeran plays guitar, and you know how Dave Grohl feels about guitars! This collaboration wouldn't exactly shock or scintillate anyone, but Sheeran and the Foo Fighters are absolutely capable of writing a single together.

Justin Bieber: Though less likely, but we kind of secretly hope it's this one. How much fun would it be to watch Bieber, adorned in his most fashionable Metallica shirt, headbang along with Dave and make his rocker dad really proud? Then all three of them would go skateboard together. Hell yeah.

Taylor Swift: Like Adele, Swift has a Kurstin connection (he contributed production and keyboards towards 1989) but appearing on a Foo Fighters record really isn't her style. She's laid low for much of 2017 and taking a break from prepping her next studio album for teaming with a big rock band's fellow fall release isn't likely to happen.

In a round-up of those who could rightfully be called the world's biggest pop star, we'd be remiss not to mention Drake, Rihanna, and Bruno Mars. But none of them have any obvious connection to the Foo Fighters, so let's close with a few outside-the-box candidates:

Katy Perry: She's got a strong Kurstin connection, as he was a key contributor to 2013's Prism. For what it's worth, she's playing Glastonbury's main stage this weekend the same day as the Foos. Given the diminished success of her recent singles, it's tough to put Perry in the class above, but there's an outside chance she's the one Dave was shouting out.

Lady Gaga: She isn't operating at her 2011 level of popularity anymore, so again, it's tough to call Lady Gaga "the biggest pop star in the world." But hey, Dave Grohl is no pop critic, and we're confident he'd appreciate Gaga's rocker credentials.

Paul McCartney: In a world where Dave is really, really getting cheeky with us, Sir Paul is the mystery man, since we could be talking all-time biggest pop stars, in which case Macca would certainly be a contender. The Foos would probably geek out to having this sort of legend in their features. But when you consider how their voices would actually sound layered on a heavy, driving Foo Fighters track -- combined with the improbability of Dave referring to Paul this way in an interview -- this is probably slightly less likely than Kendrick Lamar teaming with Imagine Dragons to make the second installment of Collision Course.