While I consider myself fans of both of these two fighters, even if Khan and Pacquiao both win their upcoming fights i just can't see either one of them getting a shot at fighting Floyd Mayweather. There just wouldn't be any reason for Floyd to accept either of these two fights.
Since Amir, barely scraped out a victory in his last fight even if he beats Devon Alexander...or whoever he fights next, I think a more likley option would be a rematch with Danny Garcia or Lamont Peterson... However if I had to pick a fighter for him to fight next I would definatly like to see him fight Miguel Cotto. Not only would the style match up make for an interesting fight but having following both of their careers it would be exciting to see the two meet in the ring.
As for Manny Pacquiao... If you beat Brandon Rios (a fighter i think many people are underestimating) I would suggest picking a fighter such as Keith Thurman or another up and coming Welterweight because i don't think I could bear to see Pacquiao v. Marquez V