Personally, I think any twist that involves the Player secretly being Xehanort this whole time would just be irredeemably stupid. Not to mention, if they were going to do something like that, they should have done it BEFORE KH3. Frankly, I think it's still a huge mistake that they didn't do Dark Road before KH3, as it would have provided much greater development for Xehanort's character before offing him instead of just retrospectively looking back at a character who should at this point be completely gone from the series equation.
But just the thought that they might actually go through with something this convoluted and unnecessary makes my blood curdle. Xehanort was a fine character as he was. Tacking on a bizarre and unnecessary backstory on top of the one he already has would just ruin him imo. Not to mention, what purpose would it serve? Is Xehanort gone or is he not? If they're going to start making connections like that to the X saga for his character, then that would imply that he'll have some importance to the story going forward, and frankly, that's a terrible idea. I loved Xehanort as a villain (despite flaws in his writing), but we were promised a definitive ending to his story. I want that ending to stay, and be left unadulterated. He got a perfect sendoff, and we've been dealing with him as a villain for the past two decades. The man is dead, let him rest, stop tacking stuff onto him as if that makes him more complex.