With the reveal in the latest chapter that Joy Boy isn’t necessarily a name but also a title, I think it’s safe to say that Roger’s words “I wish I was born in the same era as you” could mean something else than what we initially thought.
Before chapter 968, we got the impression Roger desired to be born in an earlier era, the era of the Ancient Kingdom.
But remember that during these flashback chapters it’s been shown to us that Roger is fully aware that they are far too early to be able to do anything about what they’ve discovered.
What if the line “I wish I was born in the same era as you” actually meant Roger hoping he was born at a later time, a time when he could meet the new Joy Boy and accompany him during his adventure to carry on the will of the Ancient Kingdom?
In Chapter 968 Oden said that he will open the borders of his country before the day that Joy Boy appears.
That means Joy Boy will appear in the future (he wasn’t before Roger’s era) so I think that Luffy might be Joy Boy and that the Poneglyphs also contain some sort of a prophecy for the pirate that will once again bring togther all 4 seas and that one pirate is none other than Luffy.
*Theory by Boy_Sabaw