I think the Will of D stands for two things: Dawn and Darkness
Basically there were two wills past, and both Xebec and Teach are the ones following the other will. Now there’s still a possibility that the initial D stands for a different word no one has guessed yet but what I’m pointing at is that either the initial stands for the two words I mentioned or at least could correspond to both Dawn and Darkness, a dual philosophy if you will.
Let’s take a look at the central conflict in One Piece between the World Government and Pirates. Both of these two powers follow Philosophies that could either be good or bad depending on which side of the spectrum you prefer to follow. While the World Government is authoritarian, they do protect the innocent and maintain order. On the other hand while Pirates are freedom loving some of them interpret freedom for Nihilism or Anarchy and some interpret it as living life in your own terms but having a moral code. With that in mind, Luffy and Blackbeard represent those two ends of the pirate spectrum.
Time and again the idea of Luffy, or at least a prophesized figure bringing the world to it’s dawn has been presented to us. If D were to stand for Dawn, then why would Blackbeard be a D? He represents Darkness and is the complete opposite of that.
In Christian mythology, there’s a figure by the name of Lucifer.
Lucifer is Latin for “The morning star” but other times he’s also called “The son of Dawn”. This kind of fits well with the idea of Luffy bringing Dawn to the world. Luffy also looks kind of like a shortened version of Lucifer, but let’s disregard that for a moment since that’s probably just me pushing things to fit my theory. Now Lucifer according to the mythology, was an angel (Skypiea connection) who had Free Will and who question God’s authority (World Government) and was then banished from the Heavens (Great war? Moon race connection?).
Anyway, Lucifer in Christianity is interchangeable with Satan or the Devil. The weird thing is, Satan/The Devil has been referred to time and again as “The prince of Darkness”.
So here we have a figure that stands as both a Bringer of Dawn and a Bringer of Darkness who has Free Will and questioned God’s authority. This closely follows the parallel between Luffy and Blackbeard and the Will of D. It also circles back to the central conflict of the series between Authority (God) vs Freedom (Lucifer) and the two interpretations of both Philosophies depending on which side of the spectrum you fall on. Basically, the Will of D could be a good thing (Dawn) or a bad thing (Darkness) and it depends on which will wins at the end (Luffy vs Blackbeard).
Also, this knowledge is fairly common and it would be easy to imagine Oda stumbling upon this idea while coming up with his series.
*Theory by Boy_Sabaw