I tend to agree, I am leaning more to thinking it is something about a deeper dive into Series X, whatever that may be. The term Xboxing Day just implies it is something more than purely software, at least in my opinion. So if it's not Lockhart, it might be more on Series X.
Or an Elite Controller 3.0 :)
possibly VR as well
I keep wondering if DLI came about not because of controllers but through research in VR headsets... simply because you don't notice latency issues so much through gamepads but you sure as shit notice it on VR helmets