Ok, so the theory that's being spun in my head is because of the chapter that just came out...
Blackbeard showed 2 DF's are possible... SOMEHOW. Because of the blackcloth we all have no idea how it's been done. Obviously it will be something that will need to be figured out.
Blackbeard is too powerful for Luffy right now, these DF's together confirms that... So luffy will obviously need a power of some kind.
Here comes a 2nd devilfruit into place... something that either combines well with gomu OR... Roger's old devil fruit.
I'm guessing Roger's fruit has been lost/hidden for a long time now, and luffy must stumble upon it somehow...
That would set up a great final fight of luffy vs. blackbeard.
So... any idea what the DF could be? ... or do you think luffy will somehow be able to beat him without a 2nd devil fruit? Haki alone will not be enough IMO.
I think with gaining the 2 powers the will of D has something to do with it, not the darkness fruit... otherwise BB wouldn't have kept it hidden under the cloth. interesting developments!