Davy Jones is another name for the devil or "devil of the seas" and his Locker is basically the bottom of the ocean where men go to their doom. I believe that the "devils" or spirits that inhabit the fruits are really the spirits of dead pirates/sailors who've made a pact with Davy Jones/Devil and as a consequence end up being turned into Devil Fruits. Their pact might be made upon meeting their doom or prior.
THIS IS ALSO WHY ALL THE FRUITS HAVE BEEN DISCOVERED ON THE GRAND LINE
Because the fruits literally come from the ocean, where men are most likely to die via drowning.
The curse they get from making such a pact is that even though they can live on through the bodies of men, they still lose their power when in the presence of the "Davy Jones" and are fated to return to him. This is why DF users are completely weak and drop like anchors when they are in the ocean, it Jones' domain.
A friend of mind came up with this Theory
I became highly suspicious when Blackbeard said that unlike the other Devil Fruits that existed, his fruit came from the "Devil himself", confirming that the term "Devil Fruit" wasn't merely a euphemism and that there was indeed a Devil. And since the Devil = Davy Jones, it all tied in from there. Also the fact that Blackbeard can literally draw the Devil that inhabits a user's body and render it useless reinforces the fact that his Darkness Logia is the essence of the devil himself. Which might give Teach some surprising immunities later on. (necessary immunities because of the way his fruit works.)
Another contribution was the prior introduction of the Foxy pirates' Davy Back game, where the consequence for losing was losing a crewmate, that kind of loss seemed to adhere to the pact that sailor/pirate might make.
The most important tie-in to all this is that Blackbeard-
Spoiler for Might Be: Hide
An emissary or conduit for Davy Jones. Either the fruit provides the Devil the chance to rule both the land and sea, or Teach will actually end up being a pure manifestation of his will later on aka possession. This is why Blackbeard doesn't appear so strong or intimidating now, because the time for his true awakening has yet to be seen. I also believe this might be why both Blackbeard and Luffy will make it to Raftel and the mysteries of the "Lost History" will be revealed.
