So i came to think about Inferno Scorpion since NRS said that each variation is related to some point in History, i started to think how cronological Scorpion would place in History and this is what i came up with:
Ninjustu will be the time where Scorpion works under Quan Chi and Shinnok as we saw in the chapter one of MKX he is still using his MK9 costume, but what about Hellfire and Inferno?
I imagine Hellfire will be the point in history where Scorpion's Humanity is restored, he is able to access that power trough summoning hellfire, but at some time towards assuming Quan Chi doesn't die in the Netherrealm war, Scorpion might actually Kill him and become the new king of netherrealhm, by doing so i think Scorpion's Skull from inferno variation might be from the former king of the netherrealm and now he uses to control the hell himself which is why now he has demons helping him, is just a though but that is what i think, or the timeline on a different order i mean:
Ninjutsu - Works under Quan Chi and Shinnok
Inferno - King of Netherrealm (After Death of Quan Chi)
Hellfire - Humanity Restored.
What do you guys think?