Another theory: Could the idols/mentors whatever you want to call them of the Strawhats have been members of Roger's crew?
Robin's mother could read Poneglyphs and read them all, Roger has at least seen a few of them so they could very well have travelled together.
We already know Shanks was a part of the crew.
Zeff? Mihawk? Who knows?
Zeff i've had feelings about for a while, but I don't think so. It seemed like if he had traveled with Roger he'd have been able to handle himself a bit better. He was pretty tough, but he had couldn't handle Krieg, though I suppose he didnt really make much of an effort... Iunno, I just feel that if he was tough enough to travel the full grandline he would have been able to get rid of Krieg a bit easier.
Kureha though? Definitely. Or im betting she at least met him once or twice. I cant imagine why they'd have picked up Crocus if she was on-board.
I doubt Mihawk was, but stranger things have happened.
Tom was confirmed for having met Roger, so that's a thing.
Brook had heard of Roger I think? I think he said something about Roger being an up-and-coming pirate back in his days.
I doubt Robin's mom was involved with Roger, but once again it's possible.
I REALLY doubt Bellemere had anything to do with Roger, otherwise Arlong would have been toast.