According to the manga we know that:
1. Garp is a hero of the marines that cornered Roger several times
2. Garp and Dragon have got an huge closing relationship because they are father and son.
3. We don´t know who really defeated Roger but we know that the man wasn´t Garp or Whitebeard and in the case of Whitebeard he is the stronger one.
4. Kuma seems to know about Dragon and Luffy relationship
I know that is very soon for making this type of speculations but I believe that the one who defeated Roger was Dragon in his youth after a fight that the Pirate King lost and after this he was completely defeated.
Where did Roger die? In Logue Town, a place that is in East Blue and very near the place that is controled by Garp, I believe that both of them fought against Roger, Garp was defeated but Dragon was the one who captured Roger.
I know that it could sound very lame but when Shanks said to Whitebeard "I put my arm for the sake of a new generation" he knew that Luffy was special, the son of Dragon, the man who defeated Roger has more opportunities than any other one to surpass him, this is why he gave the hat to Luffy in a similar manner that Mihawk have done with Zoro.
After defeating Roger Dragon was put in the position of the chief admiral, you must rememember that Robin flashback was 3 years after the defeat of the Pirate King and in the same flashback Sengoku is an admiral and Aokiji and Aikanu are named Kuzan and Sakaguchi (sorry if I spelled the name in the wrong way) but Oda didn´t told us who was the chief admiral.
And chronology is this:
23 years ago: Roger is defeated and captured by Dragon. After this event Dragon becomes the chief admiral of the navy
20 years ago: Ohara is destroyed by the World Government. Dragon continues being the chief admiral.
5 years ago: Dragon defected from the World Government and because his strenght and knowledge he becomes the man with the higher bounty of all.
The other part of the story is related to the Shichibukai (Crocodile in particular) and the weapons.
-Crocodile organized a revolution in Arabasta, just the same that Dragon and his comrades are doing, with the important fact that in Arabasta we can find Pluton.
-After the Crocodile incident, WG could have become very nervous and activated another attempt to make their another Pluton (Water 7-Ennies Lobby saga) but at the end they failed because instead of targeting Franky alone they targeted Nico Robin putting strawhats in the middle of the plan.
And for finishing my dumbass theory we have Kuma… why is he a robot? He is enhaced to be more stronger but he isn´t completed yet and we can conclude that if he isn´t completed then he need to be attached to something that is outside Vegapunk possibilities and I believe that this is the same weapon that Dragon and Marines are looking at the same time.