If I may interject:
Doesn't there exist the possibility that, even though it's been stated (and inferred) that Blackbeard isn't "the man Roger's waiting for", Blackbeard could still reach One Piece before Luffy? Just hear me out: What if, and I stress what if, Blackbeard beats Luffy and gets to the One Piece first. And, when he does, what if he reaches a different conclusion regarding what's there that Luffy doesn't. We all know Robin's been on the Poneglyph hunt, so she's more attuned to what they've all been saying. And, I'm pretty sure BB and his crew haven't even considered finding one Poneglyph (or, at least, it hasn't yet been stated in-manga or otherwise that he has been), so if Raftel or the One Piece is a Poneglyph (or a Poneglyph @ Raftel plays an integral role in the One Piece), who's to say BB knows what to do with it once he's approached/reached it? And therefore, comes to a different understanding of it than Luffy would.
Rayleigh made mention of the Jolly Roger pirates being "too hasty" in what they've found regarding the Void Century, and urged Robin (and, in essence, the Strawhats) to take it slower, one step at a time, so that they may come to a different conclusion. What if the "One Piece" is a relic of the Void Century, and the Poneglyph is instructions as to how to use said relic. Or something else entirely. What if BB takes this "One Piece" and puts it to use in a far different manner than "the Man Roger's been waiting for" would have used it. Obviously, the Will of D would play an important role in this, and obviously would include Luffy. The parallels between Luffy and Joy Boy, and him carrying the Will of Roger/Ace/etc. would probably play out as this goes on.
….blah blah, I could keep going on. Regardless, all I'm really trying to say is, the fight with Blackbeard and Luffy doesn't have to play out before One Piece is discovered. In fact, there could be other venues where the fight could take place AFTER One Piece is/was well done discovered and over with. Thus, the ultimate battle to come afterward could very well still be between BB and Luffy, if played out differently than to what your theories would say.
Just an idea, anyways.