I'll name the ones I consider to be the most significant:
Shinnok - Was redeemed in a rather good way in MKX after the disaster that was his MK4 version, yet after his demise he's barely ever even mentioned. Why wasn't he fighting along Cetrion and Kronika anyway? He's now what, a mere scowling head adorning Liu Kang's castle? From being the boss in three MKs and the ruler of the Netherealm?
Quan Chi - Still another MK4 combatant who in MK9 and, especially, MKX finally had a semblance of an archsorcerer, yet died and all but fell into oblivion. Shang Tsung is present & playable despite being "banished into the time void by Kronika" yet Quan Chi, who was alive and well in MK2, is nowhere to be seen?
Rain - A contender for the most shat-on classic MK combatant ever. Even if he was designed merely as a spacefiller in UMK3, there certainly was a hell of a time (24 years, someone's damn lifetime) to develop him, which they kinda did in MKA, but then shortly he was disposed off again. Truth be told, I liked his gameplay in MK9, even though he was practically disposed off even there, as well as in MKX where he's Mileena's jobber.
Shujinko - I, for one, have always liked the idea of an anti-Liu Kang. A protagonist inadvertently working for the archnemesis. Was he really that uninteresting for the NRS to return? He's one of the new gen characters with an actually deep story and motivation. And apparently Onaga and Damashi are mentioned in MK11. Does Shujinko's presumed absence in this timeline imply Kronika didn't even bother to connect Onaga and Shujinko now?
Chameleon - If mimickry worked fine enough with Triborg, if might have worked even better in MK11 with Chameleon. He was basically a "Zaterran sorcerer", or something close enough to it anyway, and I thought the idea was cool. I wish he was given the ninja-mimicking in MK11, instead of pasting it to Shang Tsung.
Motaro - Should have at least been an unplayable sub-boss, immune to the projectiles and crushing/fatal blows to make the anatomy-related things simpler. I get it, he's "too complex to design", but still... Seems to me more like the devs are too lazy to bring him back. A hell of a potential wasted, anyway. The NRS's games deserve a centaurian.
As an Ashrah fan, I'll mention her, too (I know the game is out, but maybe as DLC?). I know she got some flack in Deception for being a bit of a Raiden clone, but with MK11's emphasis on weapon moves, her kriss could be better utilized. Plus, they could also find a way to have her use her voodoo doll in combat. I also thought she could be given more light-based powers (to go along with her thinking her soul is getting purified, her moves could look more angelic)--for instance, a flash of light that blinds her opponent.
Dominik: What happened to this idea? Kano’s son could be extremely interesting from a story standpoint.
Could've swore Rey Mysterio was the father (joke).
I see a profound resemblance. Paternity test needed (joke).
You know the blade is Sonya's Blade (pun/joke).
The lowest one.
It was either nicked from Special Forces ICGU (International Crime Gang Unit) or RDCS (Red Dragon Crime Syndicate). The other knives look more Black Dragon.
"What, you use it to knife your mother in the back" (and you know the rest). Man, Kano's despicable terrorizing his entire bloodline like that. I wonder who he's talking about when he says "Nah. It put a big smile on your partner, though. Ear...to ear" when referring to one of Sonya's people.