These are some of the most anticipated people to return so I made some ideas:

KABAL
Alliance: Evil
Occupation: Tekunin
Styles: Tiger, Kung Fu, Buzz Saws

Special Attacks: Fireball, Speed Dash, Corksrew Kick
Fatality 1: Cuts opponent through waist with saws and kicks torso off
Fatality 2: Cuts through the opponent from skull to their waist

Story:
Killed by Mavado after preventing him from detsroying a hospital Kabal is rebuilt by the Tekunin to be used as a slave to his own killer.

Ending:
Used by Mavado to destroy the Earth warriors he is given one last mission: destroy the hospital he once saved. But when he arrives at the location with the bomb in hand something kicks back into his mind. He turns on Mavado killing him and for forgiveness from his dead friends he destroys the Tekunin and Red Dragon HQ with the bomb, including himself.

BARAKA
Alliance: Neutral
Occupation: Nomad
Styles: Leopard, Monkey, Wrist Blades

Special Moves: Spark Shot, Shredder, Throat Cut
Fatality 1: Classic MK2 impale
Fatality 2: Goes into a rage and rips the opponent apart

Story:
Defeated by Sonya and captured by the OIA years ago Baraka is imprisoned during the Deadly Alliance. But when Sonya goes missing Jax sends him into Outworld to find her.

Ending:
Baraka was always overlooked as a warrior and his race was considered useless. When he finds Sonya in Outworld he remembers how she humiliated him and the time he spent behind bars. In a fit of rage he attacks her and cuts her to pices. Soon everyone will know the power of Baraka and the Special Forces is next...

SMOKE
Alliance: Good
Occupation: Lin Kuei
Styles: Jutjutsu, Capoeira, Broadsword

Special Moves: Spear, Grenade, Teleport
Fatality 1: Rips off all of his opponent's limbs
Fatality 2: Stretches opponent until they snap in half

Story:
After many years lying dormant in Outworld the elusive Smoke is rebooted and given back his soul by the Elder Gods. He is told to travel east where he will meet his ultimate challenge and be able to prove himself.

Ending:
Finding Kung Lao's wounded body and Sub-Zero in the clutches of the evil Dragon King he attacks in a rage. The battle is intense and even though Smoke loses his arm in the process he is able to stall to give the Earth warriors enough time to escape through a portal. Once they are gone he sets off his self-destruct unit and it destroys the realm of Outworld completely.

SHAO KAHN
Alliance: Evil
Occupation: Warlord
Styles: Wrestling, Kick Boxing, Mallet

Special Moves: Fireball, Shoulder Barge, Shadow Punch
Fatality 1: Snaps the opponent's head off
Fatality 2: Vaporizes the opponent with eye lasers

Story:
Shang Tsung and Quan-Chi did not know that when Shao Kahn was killed his soul would be sent into his Soul Cage to be protected. After being released by the samurai Dairou he sets out to destroy his enemies for good.

Ending:
Shao Kahn discovers that Reptile has released the Dragon King into Outworld. During a fierce battle Shao Kahn makes his way through both the warriors of Earth and Outworld. He finds Reptile and kills him but the spirit does not flow into him but another warrior instead...