I'd rather see him fight Berto, even if Berto is unproven he has an undefeated record and was set to face Mosley a while ago. Mayweather dominated Mosley far too much (aside from the 2nd round) for this to appeal, as he's clearly past his best.
I know undefeated records stand for little if you haven't fought complete quality, but who knows where Berto will stand in the future, and it'd at least make a chance for Pac to fight an up and coming fighter.