I don't know what has Oda said about this (couldn't find anything) so... here's a theory- what if the will of D. is connected to the great kingdom against whom 20 others united to fight. D in my opinion can symbolize the name of the kingdom or the ruling family of that kingdom as it is known that they are enemies of celestial dragons. And they are found in all the seas so they probably escaped to all parts to survive after the war. Maybe joyboy was the ruler of that kingdom and the last island is that only, it's his home. World government tried to erase it off the map too and they could easily have lied that it was destroyed completely but a part of it remains. Joyboy is known to spread happiness and carriers of D. die with a smile on their face so it's highly likely that all 3 of these are connected.