I have no doubts he will be, but my worry is that it won't be properly portrayed by Oda.
Example: Doflamingo completely dominated Luffy throughout their fight, even after gear 4 he would have killed Luffy. Luffy won through a combination of Law, the gladiators, rebecca and practically the entirety of dressrosa helping him.
The result? Luffy was generally portrayed as ''the man who beat doflamingo'', given a ridiculous bounty increase, and then started fighting on par with yonko first mates almost straight away. We were basically expected to just accept that Luffy was above Doflamingo and could definitely take on guys who were stronger.
It would be a disappointing finale if the same thing happened at the end of one piece.