It's hard to say that anyone who's made an appearance in the last two games (either playable or cameo) is unrealistic, with the exception of maybe Stryker, Shujinko, and Taven -- the latter two because their character concepts are fully built around them being the central stars of their respective games. If a character has gotten an onscreen appearance of any sort, they're at least on the radar. So for this list I'm gonna have to reach way back there....
Kai: Because of course Kai. We need a male character who can be as acrobatic as the ladies usually are, and while Johnny is halfway there in his current iteration, Kai can really crank it up.
Kira: Get rid of the Sonya/Kano mashup B.S. and revamp her moveset, but keep the story angle where she's an anarchist making arms deals with terrorists to provoke the downfall of civilization. Kano's criminality is just business; Kira can be a straight up villain.
Stryker: Unpopular enough to still be unrealistic despite still being around in the background. How do you differentiate Stryker from the Special Forces characters? Play down the militarized assault angle and play up the more defensive riot cop archetype. You can give him a riot shield to smash opponents, reflect projectiles, reduce chip, inflict damage against incoming strikes, etc. Handcuffs can be used in parries, throws, and Fatal Blow. Though he still has a gun and perhaps some gas grenades to stun opponents, he's mostly close range and highly defensive. A shield-focused character is still an open niche in Mortal Kombat, and Stryker can fit the bill better than anybody. Bonus points if he also acquires a vicious and loyal K9 companion (or Hellhound, for Revenant Stryker).
Nitara: She may be facing stiff competition from Skarlet for the blood theme and Mileena for the "monster woman who bites people" theme, but Nitara still has something no one else has besides Onaga: winged flight. It worked for Hawkgirl in Injustice. Though her blood-related moves would have to be limited now that Skarlet is around, she can pick up other tricks like invisibility, bats, moon magic, or what have you.
Ashrah: So far there haven't been any other characters with "holy" power in the canon. Plus, voodoo dolls are a thing we haven't really tried yet. It's a disturbing and sinister kind of tactic to use when you're trying to earn redemption, which can set her up to be an anti-heroine who earns the heroes' allegiance but not quite their respect. She'd probably get along okay with Dark Raiden though, if he sees fit to weaponize her talents and willingness to push the moral line farther than the other heroes would.
