Can you please elaborate on why this is "Most likely?"
The way I see it is this...so far Boon has confirmed that the "ality" has been in another previous game, but NOT in the mk arcade era.
That leaves (if my memory serves correctly, i may be wrong):
Hara-kiri
mercy
animality
death trap finishers
create a fatality
The dark tone of MKX leads me to believe that the goofier animalities wouldnt fit.
Mercies again dont fit the dark and brutal tone of the game.
Death trap (instant kill) finishers dont work in a 2d game (they are essentially a "ring out")
And we can all agree that create a fatality was really lame in Armageddon and I doubt they want to go back down that road.
Hara kiri however fits the darker atmosphere of mkx, it adds a new element of the loser "the one being finished" still has some say in how they go out which is interesting and something that will be fun to see.
I obviously dont know for sure, only nrs does...but that's what I would put my bet on.
Just seems logical