The most accurate theory about the Will of D. in One Piece… so far.

SUMMARY

One of the biggest mysteries in One Piece that has been haunting the fans since the beginning is the Will of D.

According to a theory, the Will of D represents two different ideologies, one which helps in the prosperity of the world and the other that represents darkness.
These two wills are portrayed by Monkey D. Luffy and Marshall D. Teach (Blackbeard).

Monkey D. Luffy, Trafalgar D. Water Law, Marshall D. Teach, and Gol D. Roger. The only common thing between all these One Piece characters except their roles as pirates is the initial D in their middle name. The mysterious initial signifies a Will that is associated with these characters, and its definition has multiple renditions.
Monkey D. Luffy in One PieceMonkey D. Luffy in One Piece | Toei Animation
While some believe that the Will of D links the characters to the devil, others believe that it is a family name that has been passed down for generations. But no one knows the actual story behind it, except for Eiichiro Oda, the manga creator. Multiple theories have been devised to find out what the Will of D could be, but none of them can be termed accurate.

However, there is one theory that might shed some light on the mystery and offer a new perspective on the Will of D. According to the theory, the initial D does not have one, but two different meanings.

Theory Explains the Will of D in One Piece

u/Boy_Sabaw posted a theory on Reddit that explains the Will of D and its connection to the One Piece world. According to the theory, the Ancient Kingdom is based on the Republic of Pirates, which was a confederate of pirates that was governed by two different factions who had different philosophies.

The theory states that as the Ancient Kingdom is inspired by the Republic of Pirates, they might also have different philosophies about the idea of freedom. These philosophies were deemed the Will of D. While one represented the dawn of the One Piece world, another represented the darkness that needs to be imposed because of what happened with the Ancient Kingdom.
Robin Discussing the Will of D. | Toei AnimationRobin Discussing the Will of D | Toei Animation
The dawn of the world is represented by Luffy and the Straw Hat Pirates, while the darkness is represented by Blackbeard and his crew. Luffy followed the path of Gol D. Roger, while Blackbeard followed the path of Rocks D. Xebec, making them the ultimate rivals of the One Piece story.

Luffy and Blackbeard Represent Two Different Wills of D

Since the introduction of Luffy and Blackbeard in One Piece, both characters have followed completely different narratives. While Luffy wants freedom to explore the seas and become the Pirate King, Blackbeard wants absolute power and to take control of the whole world. The only thing that connects them is the initial D in their name.
Vegapunk Talking About the Ancient War | Viz MediaVegapunk Talking About the Ancient War | Viz Media
Their contradicting actions in the series support and add credence to the above-mentioned theory. Therefore, they were destined to follow opposite paths, no matter what, and there is also a chance that they might unite together to defeat the Celestial Dragons and the World Government, who think that they are the Gods of the One Piece world.

This would make Blackbeard the final enemy of Luffy because of their opposing mindsets about freedom. This aspect is also supported by the recent revelation made by Dr. Vegapunk about the war of the Ancient Kingdom in One Piece and his inability to decide who was right and wrong.