I'm sure most people's 'six' would look like this:
Mayweather, Pacquio, Mosley, Cotto, Margarito, Berto
But the 147lb division is so deep with talent right now there's a multitude of gap fillers should the likes of Mayweather not want any part of it. Clottey, Judah, Collazo are all there, Bradley could be brought up from light-welter, Williams could be brought down from light-middle, the possabilites are endless!
Very difficult to see Mayweather accepting any part of it, and its unlikely Pacquio's advisers would want him in it. By my gut feeling tells me the rest could very well be up for it.
And if the rest were to be in it, Pretty Boy Floyd would be left out in the cold, or faced with only one option, fight Pacquio
I guess there are too many snags though, getting Mayweather and Pacquio involved being the biggest two, then there's Showtime, they probably dont want to start another tournament until this ones done and dusted. Nothing wrong with daydreaming though is there?