'Mamma Mia' Stars Pitch Their Plan for a Third Movie -- and Cast Lady Gaga! (Exclusive)

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Here we go again...again?

With the resounding success of this summer's sun-soaked musical blockbuster, Mamma Mia! Here We Go Again -- the long-awaited sequel to 2008's ABBA jukebox musical has grossed over $392 million to date -- fans are already clamoring for another installment in the MMCU (Mamma Mia! Cinematic Universe, natch), if only because the film's catchy soundtrack is still stuck in their heads.

But what would the story be? Here We Go Again went back in time to examine how Donna Sheridan (Meryl Streep in the original, Lily James in flashbacks) found her way to the Greek island of Kalokairi and got pregnant with her daughter, Sophie (Amanda Seyfried) -- the twist being that Sophie's dad could be one of three men with whom Donna had opportunistically-timed dalliances. The film also introduced Donna's mother, Ruby (played to indomitable perfection by Cher), a performer and largely absentee parental figure, who has a complicated romantic history of her own.

"Dream plotline: We go back and look at Ruby and her journey through heartbreak and beyond," Jessica Keenan Wynn, who plays the younger version of Christine Baranski's Tanya in Here We Go Again told ET's Ash Crossan when the cast returned to Greece to promote the film's digital release.

"With Senor Cienfuegos," James added, referencing Andy Garcia's character, the infamous "Fernando," with whom Ruby had a fling in South America before Donna was born.

Another round of flashbacks in Mamma Mia 3 would mean casting Cher and Garcia's younger counterparts. But James and Wynn already had a spot-on idea as to who should play young Ruby: Lady Gaga!

The casting is pretty perfect. In fact, the pair have already collaborated on a song, "The Greatest Thing," which was a track penned by Gaga that didn't make the cut for Cher's 2011 album, Closer.

But with Ruby and Fernando's South American history, would Mamma Mia 3 still be set in Greece? "Maybe there will be flashbacks or maybe they vacation to Greece," Wynn offered.

The actresses also dished on other musical acts they'd like to see turned into jukebox musicals. "Spice Girls would be cool," James noted. 

"I just did a film that was all The Beatles music," she added, referencing her upcoming role in an as-yet-untitled comedy from Danny Boyle and Richard Curtis. "It's not a musical but it's like... there were Beatles songs in it."

"I always would say Queen, but now Bohemian Rhapsody's coming out which I'm very excited to see," Wynn agreed.

ET also sat down with some of the men of Mamma Mia! Here We Go Again -- Hugh Skinner, who plays young Harry, the role originated by Colin Firth; Josh Dylan, who plays young Bill, Stellan Skarsgård's flashback counterpart; and Jeremy Irvine, who plays Pierce Brosnan's Sam in the flashbacks. And great minds think alike, because all the actors had the same idea for their own jukebox musical: English boy band Five!

"I think we kind of like them because we saw them at a festival recently and there are only three of them now -- which works well for us," Skinner explained.

"Five is amazing," James agreed,  though she noted that Wynn might not be as familiar, "because you're American.... I don't think they quite crossed the Atlantic, but how did [the guys] know that in unison?"

"'Cause they're so tied to each other!" Wynn said with a laugh. Check out the video below to see more from the men of Mamma Mia! Here We Go Again, including which of them would be down for a third film!

Mamma Mia! Here We Go Again is available on Blu-ray and Digital HD now. 

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