Alternative Live Aid acts.

So assuming groups hadn't split or people hadn't died what could have been the star acts of Live Aid?

There was a rumour that the Beatles would play with Julian Lennon standing in for his dad.
 
If the principals are still alive, I can see the Beatles (with John, not Julian - they would never have done that) closing the London show, and Elvis closing the Philadelphia show.

More interesting are the no-shows that could have been there but weren't. Boy George was starting to get into his heroin problems at the time, and that supposedly took out Culture Club. Billy Joel and Huey Lewis had other commitments. The Eurythmics I'm not sure about. Cyndi Lauper was ill, IIRC. Prince begged off but sent a video; Bruce Springsteen was asked but declared his band needed a break after a year of touring. Stevie Wonder was also asked but supposedly said he didn't want to be the "token [n-word]"; puzzling since other performers were black. Michael Jackson I don't know about, but this was after he'd been burned doing that Pepsi commercial.

Oh, one more not-deceased: Marvin Gaye, had he not been shot the year before, would have been awesome at Live Aid.
 
Some ideas (maybe a few far-fetched)

* Pink Floyd.
A stretch, as there is a fair bit of bad blood in the mid-80s, but maybe Bob Geldof *really* turns on the charm to get this.

* The Velvet Underground
Even though they weren't a top tier classic rock band, they had the VU album of outtakes released earlier in 1985, so their profile was on an upswing. Either the John Cale or Doug Yule line-up could reform.. and Nico is still alive in 1985, so a VU & Nico reunion is possible.

The Wikipedia article on Live Aid has a 'Notable absences' section which makes for interesting reading.. acts from Bruce Springsteen to Michael Jackson, Prince to Talking Heads, Deep Purple to Frankie goes to Hollywood. Live Aid could easily have been 2 or 3 days if they all had a slot!
 
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