I believe that the Monkey D. Family (and possibly Gol D. Family) are actually Royals from the Ancient Kingdom. Where as everyone else with the “D” are simply regular bloodline.
Oda loves foreshadowing things which is primarily what this is all based off.
1. Enel’s God complex
Enel thought he was a god and when he went into his lightning form he stood exactly and even had the ribbon of lightning around his back which at the time was fitting of a “God” which is just like Luffy’s gear four.
2. Ace’s birth
The story of Ace’s birth is awfully reminiscent of the story of Mary Magdalene and the birth of Jesus’ son (if you believe in that). Mary had to escape from those in power, in this case it was the World Government and secretly give birth to Ace. We know that Oda loves to take story’s and myths and turn them upside down in a creative way.
Roger was quite close with Garp and he was certain that Garp would take care of Ace which Luffy then became brothers with. I suspect that they were actually truly related some how. Sabo joined not long after who was actually a Royal. What’s the old saying “Everything happens in threes?”
Luffy has always been told he is a complete copy of Roger by a few people. Like Roger did, he is striving to become the “King” of the pirates.
3. Whitebeard’s speech at Marineford
Whitebeard said Roger isn’t waiting for him. I think this is because Blackbeard is not a royal or actually from the family who apposed the royals. My guess is Ace’s bloodline was a royal bloodline and that specific bloodline was extinguished however, the royals are yet to be wiped out.
4. Fishman Royals
All the royals of the Fishman Island all have that same design of the flowing ribbon. Just like Enel and Luffy when in Gear 4. I find it odd that the pattern of royals all have certain aspects that are the same and Luffy is also in this category who is also a perfect candidate to be “King”.