Former US boxer Mike Tyson speaks during a news conference to announce India's first global mixed martial arts Kumite 1 league in Mumbai on Sept 28, 2018. (PUNIT PARANJPE / AFP)

Mike Tyson hasn't stepped into a ring in 15 years, when he was a mere shadow of his former self.

The once undisputed world heavyweight champion lost three of his last five fights, capped by quitting in his corner against Irish journeyman Kevin McBride on June 11, 2005.

Now 53, the self-proclaimed "baddest man on the planet" is plotting a comeback.

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I know the art of fighting. I know the art of war. That's all I've ever studied. That's why I'm so feared. That's why they feared me when I was in the ring. I was an annihilator. That's all I was born for

Mike Tyson

Back in March, Tyson opened up about the emotional struggles he's coped with since leaving the sport that made him a global celebrity.

"I know the art of fighting. I know the art of war," Tyson told his 11.5 million followers in an Instagram podcast. "That's all I've ever studied. That's why I'm so feared. That's why they feared me when I was in the ring. I was an annihilator. That's all I was born for.

"That's the reason why I'm crying now, 'cause I'm not that person no more … and I miss him," he tearfully added. "Those days are gone. It's empty. I'm nothing."

Within days of a Tyson announcing earlier this month that he was training for a series of exhibition bouts to raise money for charities, the Bare Knuckle Fighting Championship offered him a whopping US$20 million to compete in a single fight for the organization.

Last week, BKFC president David Feldman told the MMA Fighting website that his company has increased the offer and added several new incentives - including sizable donations for the charities of Tyson's choice.

"I think I know what we need to do to make this thing happen," Feldman told the website, adding that the exact value of the contract was still being finalized.

"We're not successful yet in making that Mike thing happen, but I don't think the door is shut."

Tyson finished his 20-year career (1985-2005) with a record of 50 wins (44 KOs), six losses and two bouts that were ruled no-contests.

On Instagram Live earlier this month, he said: "I've been working out, I've been trying to get in the ring. I think I'm going to box some exhibitions and get in shape.

"I want to go to the gym and get in shape to be able to box three-or four-round exhibitions for some charities and stuff."

Former heavyweight boxing champion Evander Holyfield looks on during a charity boxing event in Lagos, on May 25, 2017. (STEFAN HEUNIS / AFP)

CBS Sports and TMZ Sports have reported the list of potential opponents for Tyson includes New Zealand rugby star Sonny Bill Williams and UFC Hall of Famers Tito Ortiz and Ken Shamrock. Former rival Evander Holyfield has also been mentioned.

Tyson was disqualified for biting off a portion of Holyfield's ear in their 1997 title fight.

But Feldman said he's not worried about finding an opponent just yet.

"First, I want to see if we can make the Mike thing happen, period, and then we'll start talking opponents," Feldman told MMA Fighting, adding that Octagon legend Wanderlei Silva is also on the list.

During an appearance on Young Money Radio last week, Tyson said he's still mulling his options.

"We've got so many guys that want to do this," he said. "We're in calls, we're doing business with guys you ain't even going to believe when their names come out."

In this Feb 22, 2020 photo, British boxer Tyson Fury celebrates after defeating US boxer Deontay Wilder in the seventh round during their World Boxing Council (WBC) Heavyweight Championship Title boxing match at the MGM Grand Garden Arena in Las Vegas. (John Gurzinski / AFP)

Is it possible that current WBC heavyweight champion Tyson Fury of Britain might be one of those "unbelievable" names?

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The 31-year-old Fury, who reenergized the heavyweight division by blasting out previously unbeaten Deontay Wilder in their Feb 22 title bout, told Britain's BT Sport that he received a call last week about fighting Tyson in an exhibition.

"I got a phone call from a guy asking, 'Would you like to fight Mike Tyson in an exhibition fight?'" Fury posted on Twitter. "I was like, 'Hell yeah! But I don't think anything will materialize of it, to be honest."