The prospect of AC/DC's possible reunion naturally sparks both speculation and debate. There is still so much fans don't know, from the nature of their supposed reunion to how this updated lineup might work.

Whatever Angus Young has up his school boy-uniform sleeve, they've taken a very, very long road to this place. The Rock or Bust era left AC/DC battered and nearly broken, with Angus as the last remaining original member. Newly released pictures out of Vancouver appear to show an attempt to put the available pieces back together again. It won't be easy.

AC/DC lost founding rhythm guitarist Malcolm Young to a dementia diagnosis before Rock or Bust even arrived back in late 2014. Then drummer Phil Rudd departed amid a string of legal problems. Nephew Stevie Young took over for Malcolm Young, and former member Chris Slade returned to the drum chair, for a supporting tour.

But AC/DC's struggles continued. Singer Brian Johnson suddenly left the road to deal with a worsening issue with his hearing. He was replaced with Axl Rose, and AC/DC gamely honored their performance contracts – but not before bassist Cliff Williams decided to retire once the scheduled dates were completed.

Few could hazard a guess as to what might happen next, though there was talk that a Rose-led concert had been filmed. Johnson made a few tentative steps back, and Rudd left his troubles behind. Yet the prospects of another AC/DC project seemed to be dwindling, as Malcolm Young eventually succumbed to his illness and Rose refocused on Guns N' Roses.

That is, until Angus Young, Stevie Young, Phil Rudd and Brian Johnson were apparently caught on camera in Vancouver, where AC/DC has recorded their last three studio albums. Unfortunately, that's all we know so far. Of the things still to be determined, however, here are three big questions raised by AC/DC's Vancouver photos ...

1) Are they in Vancouver to record a new album?

No one would fault Angus for trying to get the old band back together – or at least a reasonable facsimile. Johnson's presence makes sense, too, since his hearing issues were by all accounts related to live performances. But what if this supposed reunion isn't in service of new music, at all? Stevie Young has done a credible job on stage with the old songs, but they're still missing an inestimable creative piece without Malcolm Young. A more manageable pursuit might involve putting the finishing touches on a best-of project, or a box set. It's easier to picture them completing, say, a bonus track. They might also have gathered to provide extra content for a Rock or Bust-related special edition – or even the long-rumored concert recording from Chicago's Wrigley Field. AC/DC hasn't put out a live album since 2011's Live at River Plate, which was recorded at Argentina's national soccer stadium in Buenos Aires.

2) If they returned to the road, would Brian Johnson be able to safely join them?

Johnson's sudden departure didn't signal the end of his career. Stephen Ambrose almost immediately contacted him to discuss newly developed in-ear monitors that the inventor said could help Johnson return without risking further hearing loss — and Johnson says he came away "amazed." In the meantime, he never actually retired: Johnson has since appeared on new records by Jim Breuer and Greg Billings, and he's even returned to the stage. That dovetails with Johnson's comments when his hearing issue was first revealed in 2016: "While the outcome is uncertain, my attitude is optimistic." Perhaps Ambrose's "breakthrough ADEL hearing technology" – or something like it – could play a role in getting Johnson back on the concert trail with AC/DC. Then again, there's always the prospect of Brian Johnson working as their studio singer, while Axl Rose – in a nod to the Beach Boys' approach with Brian Wilson – handles road duties.

3) With Cliff Williams' retirement, who would play bass?

Williams remains retired, if the photographic evidence from Vancouver is to be believed. And his absence isn't too surprising, considering the finality of his goodbye message. "I just need family time now, to just chill out and not do this," he said in 2016. "It’s just – I'm just ready to get off the road, really, and do what I do." So, who could potentially replace a guy who held down the bottom end for nearly four decades? There's Larry Van Kriedt, if Angus is ready to add another original member the mix. There's also Mark Evans, who was with AC/DC from 1975–77. His arrival would return all of the living members of AC/DC's T.N.T., Dirty Deeds Done Dirt Cheap and Let There Be Rock lineup.

On the other hand, they could continue to plum the Guns N' Roses lineup by adding Duff McKagan or Tommy Stinson – and either of those bassists would mesh well with Rose, should he return for live duty. (Down side: If Rose and McKagan both joined, there would be more original Guns N' Roses members in this band than AC/DC co-founders.) Of course, Geezer Butler, Michael Anthony and Geddy Lee are free these days. Butler toured with AC/DC as a member of Black Sabbath in 1977, and Lee is in Canada, too. Stone Temple Pilots bassist Robert DeLeo always seems to have time for side projects, from Delta Deep to the Hollywood Vampires. Then there's the do-anything Dave Grohl: He played bass on the Foo Fighters' debut, on 2013's Sound City Players tour and, more recently, on a 23-minute solo instrumental.

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