I think it might be Xehonort
this is probably unoriginal but what if it's a forgotten member of Organization XIII? one that was on a special mission or that left but was never dealt with. Or Xehanort in a new enemy form that has yet to be shown.--Memorized
I think it's someone we haven't yet seen. Nomura has said whoever it is is in the same vein as the Lingering Sentiment of KHIIFM+. Quote from the latest Dengeki issue: This new character must be someone involved in the story, right?
Nomura: I think it’d seem so, but at the point in time of Birth by Sleep, this person’s true figure isn’t fully understood. Similar to the way the Lingering Sentiment was, its a character who at the present time is an existence of mystery.
So that's what I'm basing this on