Hmm... For me I'd like to see Frost, Shang Tsung, Hotaru, Reptile, and Bo Rai' Cho be the last five characters returning. Of course, there are a few others that I didn't list, but these characters are definitely who I'd like to see more than any.
Frost - I am a fan of hers, and I've always made some random story up about how she could fit in this game, whether she'd be already in the Lin Kuei and escape its Automation Program to fighting alongside the good guys. I kinda disliked that they made her some delusional nutcase that just wanted the Grandmaster medallion. And it was interesting, to say the least, that she was one a few who had a defeating ending in Deadly Alliance, however, I just didn't appreciate that she did it AGAIN when she came back in Unchained. I personally hope to see her. Would I consider her being a wasted spot because she'd be sharing similar abilities to Sub-Zero? Absolutely not. She can work by having different effects to her cryomancer abilities. That is if area of effect can be played in this game.
Shang Tsung - Since there was a tweet going on stating that he's not "dead" and that Shao Kahn took only his "powers" to put inside Sindel and not his entire self, if that's really the case, I hope he comes back. Will he be powerless? I have no idea, but I certainly would love to see him come back. Heck, I really wouldn't mind if they gave him more of a Cary Takawa look, since I freaking loved him in the movie. What would he do in this game? Well, unfortunately I haven't figured out that much since well... that tweet completely confused the heck out of me. And normally I don't believe tweets, but that was just... odd.
Hotaru - I really liked this character in Deception. I thought he was probably the best designed in the game. Hotaru should have another chance to come back. However, I don't know what role Orderrealm would play in. Maybe Kotal could have something to do with the Seidan army, but then again, that's basically giving him his old story back and not doing anything new. At least he could have something more interesting happen to him rather than going up against two lame characters. Really, Dairou and Darrius = snorefest.
Reptile - Reptile should not be evil anymore. I feel that it doesn't do him justice at all. A character who wants his race back, I feel that it should finally be. Let him. After all, if this game is supposed to pass the torch, can't he have his race back and possibly pass it to one of his kind? I'm not necessarily asking for him to be good, no. I don't see him ever turning good. But I do believe that he should turn neutral, but still have his evil instincts every so often. I do like this character a lot, and I really hope that he has sort of a combination of his Shaolin Monks/2011 appearance mixed together. None of this full raptor bullshit.
Bo Rai' Cho - Last but not least, Bo should definitely have a role in this game. I do not mind for his bowl movements and vomit, it's apart of his character. But if he was to be in the game's story, he should be serious. I mean he was during Deception's conquest, he had a little humor but that's the old master kind of character. He trained Liu Kang to be who he is... er, was. Why can't we see him come back and have a bigger role in the story? He's not that bad of a character if you look pass the bull that people get too touchy about.
That's really all I have to say... for now.