Maybe uncover more truths about the Devil Fruit. Apparently Teach knew something that no one else has which allows him to copy other devil fruits. I'm willing to put money that him having two devil fruits doesn't have to do anything with his Yami Yami no Mi abilities. Why did he have to cover himself and Whitebeard to copy his ability? Anyways, they are merely speculations but I hope more about the Devil Fruits gets talked about as they enter the New World.
I want to know more about Haki.
We saw how it can create barriers of energy in the war with the 3 Admirals defending against one of WB's quakes, and Shanks used CoA to damage WB's ship a bit. Haki can also change the properties of a DF going by what Luffy can do with his rubber body, so imagine the possibilities for other DF/Haki interactions.