I've just realized 2 possible secrets we may not have found yet.
In the krypt you unlock a render of an arena called The Frozen Katakombs. The description is this:
"Ahhhhhhhhh... The Frozen Katakombs. This is the home of Sub-Zero's Ancestors, generations of warriors buried here in Outworld. I don't want to give too much away but hopefully you can do battle here in the near future. Render and concept by Tony Goskie."
That seems pretty convincing to me that we can somehow unlock this arena...
Another possibility for a secret is play as Onaga. The only clues i have for this are the fact that there is a Dragon King Storyboard in the krypt. Other characters also have storyboards, and storyboards are actually sketches of their arcade endings. Why include a sketch of an arcade ending for the dragon king if you can't even get it?