Since Mosley and Berto wont be available in time, I think the only fights which can add to Floyd's legacy are against:
Paul Williams
Tim Bradley
But I'm not sure Bradley is ready for Floyd yet. The best remaining opponent is Paul Williams, and we all know he'll fight anyone anywhere for a buck.
I don't think that their styles make for a terrific fight. It's probably an even safer fight for Floyd than the Matthew Hatton fight, at least MH has some (although limited) punching power.
There will be 24/7 shows for sure, and they will build up Paulie as something he isn't. They'll talk about a change of trainer and how his trainer trained him wrong for the Ricky Hatton fight. They'll show how he dominated Juan Diaz (twice) under his new trainer and people will get interested. It'll be a HBO PPV with a surprisingly good buy-rate but will be a very very boring fight with Mayweather winning by UD.