I think Raiden, Sub-Zero, Scorpion are definities for obvious reasons...probably Reptile as well.
Johnny Cage for obvious reasons.
Liu Kang and Kitana for obvious reasons.
I think Smoke has lots of story potential with Enenra form now.
Kenshi, he's just a cool character who wouldn't want him back?
Nightwolf, I think for the revenants he should be one of the few to return.
Maybe Stryker, I wouldn't mind.
Ermac should come back but perhaps not as a start out regular but DLC for sure, I'd like to see Kenshi help him with his encounter with Shang Tsung and free him like original timeline and Ermac becoming good, not corrupted by Shao Kahn's soul etc. Plus, Ermac is just a badass.
Noob is a must, his MK9 defeat at the hands of Nightwolf was too mysterious and vague, plus Bo Rai Cho hinted that he's still alive some where...he has to return perhaps as DLC even.
Triborg, I like his potential and it's an easy way out to return Sektor, Cyrax, Cyber Smoke and Cyber Sub fighting styles, moves etc. But perhaps this time instead of having MKX options, maybe he evolved and due to that he'll literally be a mixed bag of a character, nets, missiles, ice grenades and disappearing moves. I would love this.
Chameleon, I think he should work either like Triborg variation/Injustice 2 gear where you get to pick a preset of some kind OR have him randomly flashing like MKT version with those moves as he's temporarily such ninja color OR just have him like MK Armageddon but improved of course on a 2D engine.
Ashrah, Li Mei I think both of them should return. Li Mei for obvious reasons and Ashrah just hasn't been in an MK game in quite some time.
Taven, I think he'd be a great addition and we already have confirmation from Takeda, Scorpion's stories that he's in this timeline with more or less the same purpose, plot.
Mileena, not that I'm a "huge" fan of her but I think her character has an impact on MK as one of the more popular female characters, with her clone arc they can easily bring her back or just resurrect her some how. You know, traditional MK and all.
If they bring back Cassie and Jacqui I hope they ditch Sonya and Jax. Just please, NOT all 4...
If they bring back Kung Jin, ditch Kung Lao.
Fujin is a MUST. We've been asking for him since MK 9 and for whatever reason NRS has been stubborn not including him as a playable character.
Maybe Onaga at some point for future plots.
Tremor, I think he has lots of potential to tell.
Would like to see more Gods, Fire God, Water God, etc
D'vorah should return, she's that perfect trectorous character that everyone loves to hate and she seemed to be among the newer characters well received in MKX.
Takeda, arguably among the best newest character to the series. He should come back.
Erron Black, I can go either way with him honestly. If he came back I'd be fine with it. He seemed popular among the newer characters.
Shang Tsung, assuming Ermac's ending is canon(and of course explained better) I'm all for him coming back plus now that Quan is dead we need a sorcerer unless they create someone new, has been a while since we've had a newer sorcerer in MK.
Werewolf character, this is literally the only kind of character or species that MK has not done yet. Needs to happen.
I'd like to see Jade again, but I have a feeling she'll probably stay dead for a while unless the fanbase demand is insane for her return.
That's all I have for now.
NOTE: FOR DLC Guest characters-
Michael Myers-Boon himself liked my suggestion in the past on twitter when I posted it back during MK X before that hit. Plus if you can put in Freddy, Jason and freaking Leatherface of all characters, Myers is a win-win for a legendary horror character.
Spawn-just to see him in a really big fighting game, appropriate for his style with freedom to utilize his abilities and ruthlessness. And to shut up the many Spawn fanboys lol.
Riddick-Said it before, will say it again if they can get the rights for this character he'd fit perfectly in MK.
Terminator-Come on now, MK these days isn't exactly booming with cyborgs like the older MK games and who wouldn't want to play as freaking Arnold/Terminator?
Rorschach- Comes from a universe dark, grim af and is a brutal character himself. The REAL one that'll take a freaking meat cleaver to your head and burn your damn face off, not the poser pretending to be him now in the comics...lol
Lobo-I doubt this will happen for a few reasons, one being a direct DC property and two already in Injustice 1 but he's been in pretty messed up stories having his head blown off by Batman, limbs chopped up, blown up etc so I could see him in an MK game as one of the few DC licensed characters they could pull with little issues on the gore end.
Honorable Mentions and Considerations:
Pinhead-Would just fit in with how he kills people and theme.
Chucky-May sound like "WTF" but hear me out, Chucky aka Charles Lee Ray has two forms technically, now in the canon he can take over virtually anything with limbs or that can walk. They can have him as a Triborg/Variation kind of character where there's a human form of him and the Doll version of him thus playing like two entirely different characters. The biggest issue would be the hitbox but that can be tweaked. Plus they can create ways of making him balanced in doll Chucky form, more damage taken for one is an easy way to balance him or less damage caused. I don't think I have to list why he'd fit in theme wise if you know who Chucky is and how he kills people.
The Creeper- Anyone who's watched Jeepers Creepers knows why he could fit into an MK game.