Mayweather is considered the Lineal Champion at Welterweight and Junior Middleweight. Here is a quick definition of what that means.
To quote Ric Flair to become the lineal champ, you have to be the man who beats the man. Also, if you are universally recognized as the best fighter in the division regardless of how many belts you hold.
Sergio Martinez was the lineal middleweight champion, so when Cotto beat him, he became the lineal middleweight champ (the man who beat the man).
So, if Mayweather were to beat Cotto, he would become the lineal champion at Middleweight. How can he entice Cotto into a fight for that title? Simple, offer Cotto a the chance to be a Lineal Champ in all three divisions as well.
Mayweather holds the WBC/WBA world titles at welterweight and junior middleweight. In November of 1988, Leonard fought Donny Lalonde in a bout where the WBC super middleweight and light heavyweight belts were up for grabs. Mayweather has a return date of September 13th.
The last news that I have is that Mayweather wants to defend his two [WBC] titles in one night. If this is confirmed as an official request it will be addressed immediately. Sugar Ray Leonard did it against Donny Lalonde and before that Henry Armstrong was the only one who had that history of defending multiple titles in different divisions.
Could be a historic night, if indeed that is the route Mayweather takes.