Match a trainer with a fighter.

Wilder with Steward he had the size/length and power Wilder just needed to learn the technical side more to go along with his size and Steward would've been invaluable with that.

 

Don't know if this is exactly what the OP is looking for, but if Teofilo Stevenson turned pro after the '72 Olympics at the age of 20, him and Emmanuel Stewart would have been a great pair.

 

Freddie Brown with Bobby Chacon, Freddie Brown with Bobby Chacon!

 

Mike Tyson and Nacho Beristain. Tyson had a great career despite using the D'Amato style and keeping his feet parallel. With Beristain teaching a more efficient style, I would think MT would be much better and with Tyson's strength (and a more conventional stance), he wouldn't be getting pushed back or tied up inside.

 

Jose Medel in the hands of Futch probably never loses.

 

If time travel is allowed, I think Ray Arcel with Tyson would’ve been interesting. He was big on developing a style distinct to the fighter, based on their strengths and weaknesses, but was also big on conditioning and explosiveness.

 

A more recent one that'd be terrifying is if Tank got the crafty counter-punching and trap setting seen by most of Reynoso's fighters. That and the roids.

 

Boxing alternative history here that would be fun -- a 12yr old Wilder wanders into a boxing gym, meets Steward. Wilder had a long reign, made a ton of money, by all means, was successful. But he could have been a lot more if he had been trained right.

 

Mike Nunn with Steward at Kronk in the 80s. He would never have lost and retired out of boredom about 93.

 

Eddie Futch with Rocky Marciano. Eddie would have perfected that style for him

 

He would never have lost a fight, not even one.