If you think about it, the manga has never made any form of implication that roger did not have or did have a DF. Oda has made it a point to not mention or reveal what exactly were roger's abilities or what made him strong. Raileight did not have a fruit but other than that we really do not know much. IMO though, roger and his crew would be a lot more interesting if they had DF abilities. They'd be far more unique and would have a lot more to show other than martial arts.
Another thing, DF don't necessarily fundamentally alter a body or provide offensive abilities. The powers granted by a DF can be of esentially any kind. In that sense, it is also possible roger had a fruit but it did not change his body nor it increased his fighting capacity in the same manner it would do buggy or WB. Something I was thinking is that he could have been a "sensing man". Basically a fruit that takes his 5 senses and enhances them to ridiculous levels. It'd fit the description given by raighleit by would be extremely useful in battle and if used with observation haki it could be very useful. Enhanced senses would allow him to react extraordinarily well in a fight and perceive things others normally would. It would also allow him to literally experience life in a way no other person could. For someone who rejoiced even in his death, a fruit like that would be extremely fitting since he'd be able to enjoy things like food, alcohol, sex, sightseeing, pain and a number of other things to a degree no other person ever could have. As I mentioned before, this fruit coupled with observation haki could also be potentially useful to a great extreme. If observation haki counts as a sense, then he'd also be able to use it in a way very few others could, perhaps even better than what enel could.
