In chapter 0, during Roger’s execution, we see several notorious pirates present in Loguetown like Shanks, Buggy, Doflamingo, Mihawk, Moria, Crocodile.
We also see the infamous Monkey D. Dragon before he got his tattoo.
Why would he have any interest in the execution of the Pirate King? He is the only one with a rather concerned and serious look in the panel. (You might say Mihawk as well, but Mihawk seems like he is just merely observing)
Along with Buggy and Shanks, he is the only one who seems a bit worried or sad for the execution. Not entirely tearing up or sobbing like Shanks or Buggy, but it’s almost as if he were concerned or – at the very least – understood, to some extent, why Roger chose to be executed.
Is it possible that he may have spoken to Roger at some point before his execution? Maybe Roger recognized him as Garp’s son and a fellow member of the D.
I think that Roger and Dragon definitely met. I believe that Roger may have spoken a bit about the history of the World and the Poneglyphs with Dragon – seeing as that he trusted Garp very much and that he was also a D. After the execution, I believe Dragon was inspired to start the Revolutionary Army.
Dragon has a very good sense of justice, as we saw in the ASL flashback, when he notes that Sabo thinks just like he thinks about the status of the nobles and how society worked.
In the Reverie flashback of Vivi, we saw that Dragon had just began to be recognized as a threat to the World Government, so he couldn’t have been a Revolutionary at the time of Roger’s execution.
In chapter 100, as Luffy is spared from being executed by Buggy, we see Dragon with a rather badass self-monologue.
“That which cannot be stopped: inherited will, a man’s dream, and the flow of time. As long as a man continues to seek the answers to freedom, these things shall never be stopped” – These were the words of Gol D. Roger. I believe Roger spoke these words DIRECTLY to Dragon, before having said it to the public in his execution.
-Inherited Will: I believe Roger may have told Dragon about the lineage of the D. Just like he told Whitebeard about it.
-A Man’s Dream: I believe this is a reference to what the Roger Pirates learned about the Ancient Kingdom and the Will of their leader – Joy Boy.
-The Flow of Time: It has been 800 years since the fall of the Ancient Kingdom and the end of the Void Century – only time will bring these things together in motion.
Although I don’t think Dragon knew the entirety of the Void Century as Roger, Rayleigh, Scopper Gaban and the others did, I believe Dragon’s learning of the Will of D. set out on a mission to overthrow the World Government and their injustice to bring it True Justice and Freedom.
That is why Dragon got those Shandian-like tattoos on his face – maybe as a sign of his lineage to the D. Clan and the Ancient Kingdom – as we know the Shandian’s were, at the very least, an ally of Joy Boy and the Ancient Kingdom.
*Theory by Raymar07