Like every boxing enthusiast in the world, I would of course love to see Floyd take on Manny for welterweight and pound-for-pound supremacy. However, we all know that the probability of this fight actually coming together is extremely low, considering that Manny's camp could not even agree to a two-week cutoff on the blood testing, and on the contrary seems insistent on a 24 day cutoff.
Other options at welterweight for Floyd wouldn't satisfy either the ambitions of Mayweather or the boxing public. As much as I like Berto, he clearly lacks the skill to seriously threat Money's undefeated record. There also seems little doubt that Mayweather would box circles around Margarito. Paul Williams is a name to consider, but if Carlos Quintana could outbox The Punisher, I would have no choice but to assume that PBF could do likewise.
This of course leads to the possibility of Sergio Martinez fighting Floyd. Sergio has similar attributes to Pacman in his southpaw stance, athleticism, and speed that could give Floyd a serious challenge. His height (5'10'') and reach (76'') would also make him even larger than Floyd's biggest challenge, De La Hoya. Ultimately, Sergio combines the size of a DLH with the aggression and punch output of a Castillo to pose a serious threat to Floyd's zero.
Also, while Floyd was apprehensive about fighting for a "dust-collecting" welterweight belt strap against Shane, Martinez possesses The Ring, WBO and WBC titles at Middleweight, something that Floyd would certainly be more interested in possessing, in order to bolster his legacy and his claim to be the best ever. Should Martinez's strap at 154 be insufficient, I'm sure Lou DiBella and GBP could work out a 156 or 157 catchweight for a Floyd-Sergio fight, with one or more of the aforementioned middleweight straps on the line for Floyd to become a titlist in his sixth weight class.
In short, while Paul Williams is certainly an option (assuming a win over Cintron this Saturday), I personally believe that, without Pacquiao, the fight to be made with Mayweather is Martinez. Size is an issue, but it appears that Floyd was atleast 155 last night against Shane, and while he can continue making 147 with ease he could perhaps be not adverse to forgoing a weight drain and fighting at a higher weight against Sergio. Sergio's southpaw stance, speed, and status as Middleweight Champion also offers Floyd an opportunity to cement his status as an all-time great, while giving us enthusiasts something to look foward to.