(The coach can be from any timeline and the fighter can be from any timeline). I would make Cus D'Amato the coach of David Tua. He would not only get Tua into fighting shape as always but he would also discipline him. Unlike Mike Tyson, the protege of Cus, he wasn't out partying, getting high and hanging out with chicks. But Tua began to gain alot of weight after his loss to Ike Ibeabuchi, which also demotivated him. He entered the Lewis fight with a ribcage injury and was 20lbs overweight (in contrast to his weight at 225 against John Ruiz) and was, at his highest, 251lbs. If Tua was under the tutelage of Cus, I think he would've been heavyweight champion, perhaps unified or undisputed.
Note: While Tua was with Lou Duva, he left after the Ibeabuchi fight, so did Ronnie Shields and Tua was then under Kevin Barry, who beat Evander Holyfield in an Olympic bout only because Holyfield knocked Barry out after the bell. Barry and Tua had some legal trouble which took so long to resolve that Tua missed the opportunity of competing in a WBA title eliminator with Hasim Rahman (i believe).
Being from Philadelphia my first thought would be getting Matt Franklin (Saad Muhammad) away from the incredibly inept Sam Solomin and replacing him with George Benton.
Demsey xD’amato Foreman xManny Steward, would be interesting how he refined Foreman’s long distance punching, jabs and fights set up behind it, which was something Foreman could do and did get better at post Ali fight. Vitali xManny Steward, I have no idea why he wasn’t trained by him since his brother was and it’d make sense stylistically.
Deontay Wilder and Emanuel Steward would be a banging combo. I think Steward could have made Wilder a heavyweight version of Tommy Hearns.
It would be interesting to see how Floyd Patterson's career would develop without Cus D'Amato, with a more aggressive, less conservative coach. Many "what ifs" in this one: - Would Patterson look for Marciano? - How long had been Patterson's reign as HW champ? - There would be some intention to duck Liston ?
Holyfield and Cus would have been interesting, Holyfield for sure would have been disciplined and humble enough to not go astray like Tyson, I think he had a good build for the style too, potential for more muscle mass than Patterson and not as bulky as Tyson, more than athletic enough