Hi all, new here, thought i would chime in.
Crocus is definatly a canidate, he says that he is around 70, if he stayed there the whole time with Laboon that would make him about 20 when Laboon first arived, he also stated that he was a Doctor on a Pirate ship. So when exactly was he a pirate if not after Laboon arived at the entrance. Also remember what he said after the straw hats were out of ear shot, this might be the pirate we have been waiting for, eh Rodger?
Also Rodgers was an idiot like Luffy, as mentioned by Kokora, but in skypiea there was ponyglyph writing next to the actualy ponyglyph that said that Gol D Rodgers had been there, and also gave Robin the idea that all of the ponyglyph are tied together. So Rodgers had someone on his crew that could read the ponyglyph, this would have to be either Olivia or the unnamed father, i pick the father becuase Olivia was in the procees of finding the ponyglyphs 2 years after Rodgers was executed.
Kureha is also a posibility, she didn't have to be the ships doctor, that could have been aquired when she arrived at Drum, I would guess she was the theif.
The greater question that comes out of all of this is, if Rodger was the pirate king, and Rodgers was only equaled in strenghth by whitebeard, why in the world would he let himself be executed in the fasion that he was (unless he was a DF user, then it would make sence if he was shackled with the seastone, but if this were the case, that fruit was it, also the seastone seemed to be a new thing because Buggy seemed never to have come across it since he was shocked in Loug Town by the net, we all know that he is fairly will travelled so you would think he would know of that being a DF user himself for a long time). My theory is that the WG had his nakama and gave him two choices, die or they die.