Okay, so Whitebeard is the Strongest man in the world, Post Mortem Roger. Mihawk is the current Best Swordsman in the World.
Now, Mihawk is fairly young, probably about Shank's age (Late 30's, early 40's). Whitebeard looks to be about mid 50's - 60's.
Because of the implications of becoming Pirate King, Roger most likely had to best every great opponent of his decade. Mihawk too, had to best the greatest swordsman of the time in order to gain the title.
So I got to thinking about all the people that have spoken openly about Roger, and I wonder if, rather than being a part of his crew, they were his opponents?
Let's look at who we have:
Zeff: A strong pirate with legendary fighting skills. This one is difficult because Zeff's tenure on the Grand Line was long after Roger had died. But Zeff's age fits; perhaps he was an opponent of Roger's while both were in East Blue?
Crocus: Crocus has been with Laboon for 50 years (supposedly). I believe he says he's 73-74, so what was he doing in the first 23-24 years of his life that earned him control of the Twin Capes Lighthouse? Roger was probably a kid when Crocus was this young, but from what we see of Young Crocus, he's pretty fit. Perhaps at one time, Crocus played gatekeeper to the Grand Line, fighting whoever entered, and only those that beat him could pass? It's plausible, but then again, maybe not.
Kureha: She's 120 years old. IIRC, she states she was returning to Drum Island after having left for a few years to Hiruluk during Chopper's backstory. Perhaps she was asssisting Roger, or perhaps she was just trying to hinder him and failed. Though in Kureha's case, I think she actually was his nakama.
Hiruluk: Former thief who saw a mariacle and came back changed. Perhaps in addition to the sakura trees, Roger helped him change his ways?
Crocodile: Now, here, I believe Crocodile was a cabin boy on Roger's crew like Shanks. A kid with big aspirations. But either through bad luck or something like a Davy Back fight, he lost everything and lost his spirit.
Belle-Mere: This one is tough. Bellemere participated in a battle 17 years ago, but before that, who can say? Maybe she was part of Garp's squad back in Roger's day.
Tom: Well, this one is obvious, but Tom was a nakama, not an enemy. But who knows? Perhaps Tom made Roger fight him before agreeing to build the ship; after all, Luffy fought Franky.
Gan Forr: Perhaps Roger wanted to test his strength against "God". I bet Gan Forr was a pretty badass fighter in his prime.
Zoro's Sensei: This is for Mihawk. Perhaps Zoro's Sensei was the former "World's Greatest Swordsman", and Mihawk took the title from him? He did, after all, teach Zoro the techniques that were key to becoming greater.
Okay, now to the obvious ones: Garp, Sengoku, Whitebeard. Of these three men, who was the FINAL opponent for Roger? Most of you instinctively say "Whitebeard", but if that was the case, why would Whitebeard not have One Piece 22 years later? If he was the final obstacle, he would have been able to reach Raftel and confront Roger before he could claim the final treasure. Which is the reason I think it was Sengoku, if I had to pick from those 3.
But then, one must wonder if there was a former Pirate King: A man unsurpassed by anyone until Roger came and finally defeated him. Now, unlike my other theories, I'm not even going to suggest it's something silly like Davy Jones or whatnot. A living, breathing, flesh person fought Roger and that was Roger's final opponent. There can be no argument that this final battle took place on Raftel. If there was indeed a former Pirate King, then there's no question that Roger was Pirate King after his defeat. If he was simply the World's strongest man, then his defeat by Roger gave him an uncontested title.
But that makes it too easy. If Roger and Whitebeard could tie, then Whitebeard could have defeated this "World's Strongest Man" just as easily as Roger. So no, this man had to be something more than just that. A former Pirate King.
And when the story gets to the New World, we may just find the truth about the Pirate King before Roger, if one truly exists.
But I think it does.