Floyd Mayweather Jr. defeated his greatest rival, Manny Pacquiao, on Saturday and has nothing left to prove in boxing. Underlining that point is Mayweather saying he will vacate all of his belts-including the WBC title at 154 pounds-so that younger fighters can go after them. He could retire now, but his nickname is "Money." He plans to finish his contract with Showtime by fighting this September.

We outlined the three best possible opponents here, but now it's time to think outside the box. Here are some candidates that don't jump to mind, but could be intriguing challengers.

Lucas Matthysse (37-3-0, 34 KO)

Matthysse engaged in a Fight of the Year candidate in April vs. Ruslan Provodnikov, and won the 2014 Ring Magazine Fight of the Year against John Molina Jr. Matthysse has a knockout rep similar to that of unbeaten middleweight champion Gennady Golovkin, except Matthysse fights at junior welterweight-a weight division below Mayweather's. Matthysse isn't the most famous fighter in the world, but hardcore fans adore him, and casual fans might remember him from the undercard of Mayweather's 2013 bout vs. Canelo Alvarez.

Mayweather would likely outbox Matthysse, but "La Machina" won't stop coming.

Andy Lee (34-2-1, 24 KO)

Mayweather's been criticized for his careful matchmaking, making a bout with Golovkin all but unthinkable even at 154 pounds, where Mayweather holds a title. But maybe Mayweather wants to make a little history on his way out of the sport. Perhaps Mayweather vacates his belts, but goes for Rocky Marciano's 49-0 record by competing for the WBO middleweight title against Andy Lee, making him a champion in six weight classes.

Lee has a lot of power, but he isn't a particularly skillful champion. He recently fought Peter Quillin to a draw, and in his two previous bouts he was well behind on the cards before landing knockout blows. It's hard to believe Mayweather would be chopped down by one of Lee's counter shots, and Mayweather's made a career of making larger, stronger opponents look silly.

Manny Pacquiao (57-6-2, 38 KO)

Before you run in horror, consider this. Pacquiao is claiming he fought with a severely injured right shoulder, and that he was sabotaged by the Nevada State Athletic Commission. This fight generated the most revenue of any fight in history-ever-and a rematch would still be big money. And who else is left to fight really? Pacquiao may be finished as a pay-per-view attraction unless Mayweather is involved, and if the promoters can convince the fans they were only underwhelmed because Pacquiao is a warrior who was fighting hurt, they may give him a second chance.

This is unlikely but hey, we're thinking outside the box.