I have an idea about a challenge tower mission. I call it "End Game Nightmare".
It would take place in Kahn's Koliseum. Juliet Starling (Lollipop Chainsaw) would be tied up in the background where Kitana/Tanya was in MK9 and Rayne (Bloodrayne) would be tied up where Skarlet was in MK9. Kahn would be sitting in his thrown. The player would face off against Kratos first (for Playstation owners) or an Xbox exclusive character. Next, the player faces off against Jason Vorhees. Then, the player faces off against Predator. After that, the player fights against Alien. Now the last battle will have three parts. It starts out with Shao Kahn jumping off his thrown and facing off against the player. Once he's defeated he will morph into Shang Tsung, who will be a lot trickier because of his speed and the fact that he can morph into any MK10 character even during a combo. Just as you think you won after defeating the sorcerer, he will morph into Freddy Kreuger and will be a lot faster and stronger. Once Freddy is defeated, a special brutality sequence will occur and Freddy will explode (like how it ends with Shao Kahn in MK9). Beating that mission players will unlock Kratos or an Xbox exclusive character, Jason Vorhees, Predator, Alien, and Freddy.
Juliet, Rayne, Jackie (The Darkness), and a few more other characters as DLC.