@Helter:
Loved the theory. The part about the devil fruits possibly having the powers of dead people's souls made think a little bit further. Ever since Rocinante mentioned the fact that D is God's natural enemy, I started thinking D means devil. I believe there was a great war during the Void Century between the Celestial Dragons and the inhabitants of the unknown ancient civilization that was destroyed. Maybe these people had weird powers and the Celestial Dragons (calling themselfves Gods) started to refer to these people as devils. When they lost the war, their souls were transfered to the fruits that grew on the Adam and Eve trees, so that their will could be inherited by whoever ate the fruits, through their original powers. That's why the fruits were named Devil Fruits (I know they're named Akuma no Mi in japanese, and that doesn't have anyting to do with the letter D, but stillā¦ it could be). The only thing that doesn't make a lot of sense here is that fact that anyone can eat Devil Fruits. Even "Gods" (like Doflamingo and Rocinante) can eat them. So the whole inherited will of the Devils kinda makes no sense here...
One problem there is the bolded. People have seen the Adam tree and the Eve tree, and we've been told what properties they have, none of it is devil fruits.
Treasure Tree Adam - wood is hardest in world, able to withstand cannonballs and gunfire.
Sunlight Tree Eve - colossal tree whose roots reach down 10,000 feet to FI, through which it bring sulight and air to the island.
Usopp noted the similar names (Adam, Eve) and wondered if there was a connection.
Pity Robin hasn't told Usopp that there was the tree of Knowledge on Ohara, it housed a library that spanned 5,000 years until Marines destroyed it.