Oda's favorite Manga is DB, right?
So here's a thought:
Someone (Probably Sengoku) will find a piece of Posiedon or Pluton (The god, not the warships) and fuse with it to become god-like. Faced with no other choice, All three D's will combine into one person. The fight will be so intense that a bright light will cover all of Raftel. When it clears, Sengoku and the D's will disappear from the One Piece World.
The D's all contain personalities needed by a god:
Luffy: Ambition, Determination, Focus, Control, Acceptance
Ace: Ambition, Apathy, Pride, Loyalty
Blackbeard: Ambition, Compassion
Saul: Compassion, Ambition, Determination, Loyalty, Optimism
Basically, Roger ate the Kaku Kaku no Mi (Divide Fruit). This allowed him to Divide himself into 4 people. Whenever a hero is needed, this people will receive one. The ancient Kingdom's legacy to the world was the Devil Fruits, which I believe that they themselves became. What Roger found at Raftel was amazing power and wealth in the form of "cheating death".
Just my two cents.