Gol D. Roger is a really powerful pirate no doubt since he became the pirate king but i think he did it with out the help of a DF, and what he used was haki for combat. Lets not forget though
even though you do become a genius if you eat the human DF it does not mean you become god or have powers close to that, but what is interesting is that he has the power to hear all things, I
think is what Rayleigh said so idk he might have ate the fruit but i doubt it i just think he's pure strength, and has an insane mastering over haki.
He possessed the first mentioned abilities and a df. but think about it, this df would be greatly feared because of it belonging to the former pirate king. it would be highly sought after due to the infamy it has. someone with great ambitions would need this fruit to pursue his goals and protect himself from those who fear these pursuits. someone who if the marines found out he had this fruit would stop at nothing to capture him. someone like dragon. it is believed that dragon is a wind logia who can control the weather. remember edd war and shiki? how did roger and his crew survive against the massive numbers and ships controlled by the same guy who fought both garp and sengoku only to be the first to break out of impel down?
We don′t know nothing about him. If a DF respawns after the user dies we might see Roger′s DF ( that′s only in roger was in fact a DF user). If an enemy appears with a HAx df power that roger had everyone would piss themselves. : D it would be cool.