It pretty obvious the names he would not have fought - Leonard, Hearns, Benitez, Duran, Pryor. He ducked the most established guys at the weights he fought at after the Castillo fights, and none of them other than Pac can hold as candle to those 80s guys in terms of ability and greatness. No way he fought any of those guys.
Muhammad Ali, duh. Mayweather said if he took PEDs he would knock out heavyweights, well in the 70s and 80s everyone was juiced up and no one cared. He'd duck Ali worse than he ducks the IRS.