if cotto knocks the crap out of pacquiao and puts a certified hurting on him, no way pacquiao comes back for a rematch. in fact, if cotto destroys pacquiao (which he will), pacquiao will never fight at welterweight again. he'll have to face his feared menace (marquez) otherwise he'll be seen as the coward that he is!!!
Cotto has an OK chance, Floyd has a better chance, and Mosley probably has the best because of his strength and combos.
Cotto has about as much chance of beating Pac as Ricky Hatton does in a rematch. Mayweather looked awfully good so I'd put Pac-Mayweather at 50/50 right now. After he KOs Cotto, Manny has to go through Mayweather -- probably twice as the first fight will likely be close. If Pac can do that I think he might retire. Mosley will be too old by then. Not sure anyone would want to see Pac-Marquez after JMM's performance against Mayweather.