I believe many of these characters had the curse of first appearing in the 3D era, a time when the games were not that popular and the gameplay was shit. I mean, I loved Ashrah, but her specials were ridiculously bad, same story with Drahmin. I strongly believe Kira would've been far better received had she not had a mix of Kano's and Sonya's moves, as well as I think Fujin, Tanya and Reiko greatly benefited from a good redesing in MKD/A while Jarek and Kai (and Striker) had a shitty one that hurt them even more. Chances are, if any of this characters had been born in this game they would have a much bigger following than they do, based on having a better way to show their personality, having more detailed/better costumes and, most importantly, a better gameplay. I could be wrong, but I think most characters on this list could be redeemed. Having said that, here is my list:
Sareena: having ties with Bi-Han, Kuai Liang, Quan Chi, probably Ashrah, and now the special forces, it's not hard to find her a spot in the story. Her desing is cool, and had a cool gameplay, and both of them could be greatly improved with the use of her demon form. I knew it was a far chance, but I expected her to be playable in this game. There is a little hope if they make a second DLC pack.
Havik: As others pointed out, Havik is the ultimate MK wildcard. Because of his nature he can be added to any story as a enemy or temporal ally, always as a puppet master trying to create chaos. And as well, because of his nature, his gameplay could be anything really. If there is a second combat pack, I feel he has a high chance of being in it.
Fujin: The wind god. I don't have much to say about him that other people haven't said already. I think he could be added as DLC even if there is not another Kombat Pack, just because there is a high demand for him.
Ashrah: While her gamplay should be completely reworked (horizontal spin? Really?), the idea of light powers do sound cool, and mixed with her design, her magic sword (which is actually kinda manipulating her), and the fact she is a demon hunter who used to be one of Quan Chi's assasins, well... I think all that begs for a revisit. She was one of the few cool additions of the 3D era, I strongly believe she should return.
Hotaru: I feel he's a natural ally to Kotal in MKX, and should've had a cameo or at least a mention to show that. He is that kind of villain who is not actually bad. He had the single best costume concept of the entire 3D era, his armour is a piece of art. Sadly, the gameplay was not on that level, it would need to be reworked, but he I believe he should be reintroduced.
PD: I always was curios as to why people think Hotaru and Havik belong together, as a "we can't have one without the other" kinda thing. Yes, they serve opposite realm and forces, but their stories were independent of each other, hell, Havik is being a mayor player in the comic and Hotaru is nowhere to be found. If NRS made them N1 enemies between each other as a Sonya/Kano thing it would make both of them quite dull to me. Please keep them independent of each other.
Kai: Only if they base him more in MK4 than MKA. His fireballs were amazing, his handstance was a cool concept and his design was unique. The sad part is he would be old as fuck by now, but hell, so is Kung Lao and he look better than ever, so why not give Kai a chance? I would love a proper take on that stance, maybe even give him different ones. I never understood why people disliked him so much.
Kira: She is a bad-ass, a real one. I'll repeat it, had NRS given her a unique moveset she would've been much better received. The only cool Black Dragon member besides Kano.
Blaze: Yeah, I went there, but I speak only of his small form. Even with the generic moveset he was a really interesting character back when he was that mysterious fire elemental with an unknown mission to carry. Yes, he is the less likely to return as it would also mean to revive the Armaggedon storyline in some way, but giving him a small change, making him a "strong warriors hunter" kinda thing instead of just making him monitor the realms... I think it might work.