So I was thinking one day about what made Roger so unstoppable. I thought he has to have a devil fruit because there is no way he could've been so feared with haki alone.
I started thinking what fruit he might've had, first I thought a water manipulating one but that just destroys the canon. Then I thought back to the mera mera no mi: when ace died it just reappeared again, so when Roger died his must have reappeared as well.
So I thought about who alive could have his fruit now. I rewatched his execution and noticed it was raining, not just raining, but storming. This also happened when Luffy was about to be executed at lougue town. Who was there to conjure the storms and the wind? Dragon. Dragon has the powers to conjure winds and create storms.
So if this weather happened at both these times I came to the conclusion that Dragon has Rogers old fruit. The ability to conjure storms. Dragon was also there at Rogers execution 24 years ago. This is how he must have learned about it. Maybe dragon had the power back then as well. But you have to remember that it also stormed when Roger fought Shiki. That is no coincidence. Conclusion: Roger had the power of the wind and storms.
Mieux. 