So I'm letting my mind wander freely as a nice, hot shower works its magic this morning, and this sudden conviction appears unbidden: Mary Morstan just has to be Moran in the BBC Sherlock universe.
Moran, who only shows up in the Canon after Moriarty is gone. Just like Mary did.
Moran, who waits patiently for Sherlock Holmes's return so he can shoot him. Just like Mary did?
Moran, who . . . well, of course, I immediately Google this thought to see what fandom has come up with on the subject and no sooner do I type "Is Mary Morstan . . ." than Google supplies "Sebastian Moran." This is ground that has already been well covered, of course. There's even that coloring book theory that came up a while back.
She's definitely not the Mary Morstan we know from Conan Doyle. Her A.G.R.A. is not the fort a stolen treasure came from. She doesn't meet John during one of Sherlock's cases. And based on Reichenbach timelines, she fits rather perfectly into that Moran-shaped hole the Sherlock Canon left open. And those initials . . . "Air-Gunned Ronald Adair?" Makes more sense than "Alice Grace Rucastle Abernetty."
But we've seen the shocking-twist Canonical character fusion of Irene Adler and Professor Moriarty in CBS's Elementary. It seems doubtful that Sherlock will play that somewhat heavy-handed card with it having been used so recently by the show's American counterpart. And why waste Sebastian Moran as a part of a character we already have?
Thinking about Mary Morstan's place in BBC Sherlock's Holmes mythos, she (and her child) seem more likely to go on to be "my own sad bereavement" to John Watson (found in "The Adventure of the Empty House," just as Moran was), not because her past holds the role of Sebastian Moran, but more likely because it holds some leverage or obligations that will tear her permanently away from her marriage to Watson. Her past was not just contained on a thumb drive or in Magnussen's mind palace . . . there is still surely enough of it out there that a true Moran could use it to heartbreaking effect.
We're seen nor heard nothing, as yet, of one Captain Arthur Morstan, Mary's father. He was a a pretty life-altering figure in the Doyle Canon, and his appearance on the BBC show could hold some . . . wait a minute . . . they didn't do The Sign of Four, even though Mary has appeared, did they?