I occasionally wonder if there is too much emphasis on SU characters when in fact secrets are as simple as extra death-traps and stages.
Logically Frozen Catacombs would be an unlockable secret arena and perhaps one or 2 of the Konquest fighting arenas such as the Netherrealm.
Beyond those I wonder what deathtraps could be unlockable, assuming they were.
I think the Dragon in Dragon Mountain is an obvious one.
So are the gears at the bottom of the Chamber Of Artifacts.
How about the red wall piece with beetles inside it behind the stage weapon at the bottom section of the beetles lair?
How about the Sky Tower levels mentioned in Kontent that are seemingly inaccessible?
Could you be flung into the portal?
There has to be more to Liu Kangs tomb, what about breaking the side walls somehow?
Under the bodies on the Nethership interior?
Before anyone says it, I do realise there may well be nothing else but at least some of these things I mention were intended at some stage.