If Floyd can get past Mosley, aside from Pacquiao (which seems pretty unlikely at this point) there aren't many other choices of opponents available. Berto is the only elite WW outside of Floyd, Pac and Mosley, but he hasn't done anything to prove that he's ready for the big three (actually, he's kinda proved that he's not ready). However if he fights Cotto and somehow finds a way to win, then a fight between him and Floyd would be demanded.
But if Berto-Cotto doesn't happen, or Berto loses (which is likely), should Mayweather move up to 154? There are some pretty good names at 154, but they haven't done s**t recently. Perhaps in the next 12 months they could do something big. For an example if Yuri Foreman beat Cotto, and then went on to beat Cory Spinks (or if Spinks beat him), I'd have no problem a clash with Floyd. Or if Cintron somehow found a way to beat Paul Williams, a fight between him and Floyd would be huge. However, a fight a certainly would NOT want to see is Mayweather-Cotto. That fight has absolutely no meaning at this point, unless Cotto pulled a lucky Sanders and KO'ed Pacquiao at 154, and I wouldn't EVER bet on that happening. I'd actually bet on Pac stopping him by the end of the 6th.
There won't be much at Welterweight for Mayweather, but moving straight up 8 lbs will be dangerous, and I doubt he would do it. As for Berto vs. Cotto, Berto would toss Cotto around like a ragdoll. Unfortunately, the current Cotto is less than a shadow of what he was in his undefeated prime (i.e. until he faced plaster-equipped Margarito) - that Cotto would have made a mockery of Berto (and also of Pacquiao, in my opinion), and would easily get beaten by Berto. A further loss to Pacquiao (soon after the career-ending loss to Margarito) has ended him once and for all. Berto vs. Mayweather would then be a fine option, but Berto will get owned. It would be an interesting fight.
Mayweather could try moving up to 154 lbs, where there will be plenty of challenges, but they would be dangerous opponents and I'm not sure he'll want to do that.
If Bradley extinguishes all Junior Welterweight opposition by taking out Maidana, Khan, Malignaggi, and then serious undefeated opponents like Alexander and Olusegun, then he will be set up perfectly for a big fight with Hatton, which he should comfortably win. Then, finally, Bradley will be a big name in America and Floyd will take a fight with him (Bradley has said he will move up if Floyd agrees to it).