Well I dont think Liu Kang should become the hero again. Needs to be someone new IMO.

I think the best bets are Kung Lao, Sub Zero or Kitana.

Kung Lao, because he seems to be the main Earthrealm guy at the mo, so seems obvious he would fill Liu Kang's shoes.

Sub Zero is gaining great power etc because of his ancestry/dragon medallion etc, so it seems plausible.

Kitana is like 10,000 years old. Will no doubt have a lot of knowledge about her foes, and I agree a female should perhaps take the role as hero (or heroine). She is a skilled fighter/assassin and has brains as well as braun.

Plz your stupid flames wont work on me. And dont show me a video of him, he still sucks.

I say sub zero. It makes no sense to build him up with all the power ups if he does not do anything truly pivotal. Besides, most people can see that the situation in the netherealm involving noob (who IMO will get a big push soon), Scorpion, hopefully quanchi and subzero is going to take precedence in the next one or two games. The only clear-cut hero in there is subzero, and it is quite clear to me that something is going to happen to scorpion that will change his storyline direction forever.

MK always has dual storylines running at once, and I wiould pick Lao to be the other hero, but as the netherealm situation takes emotional precedence, sub will be the main hero.

I think a lot of characters have become stagnant over time, and need a change up; people change. I fully approve of Scorpion's Emperor-Championing so long as they keep him the brooding, not-really-good anti-hero. So I don't see him as the 'hero' per say.

People who I predict:
Kung Lao: Obvious choice for a replacement.
Sub-Zero: He's been groomed for it since MK3 or 4.

People Who I'd really like to see as heroes, but probably won't happen (yet)
Ermac: Everyobdy's already said all the reasons.
Reptile: He's just a guy trying to bring back his race. If someone of good alignment (say The Elder Gods) made him the offer, he'd make a great semi-hero.
Baraka: He's unpredictable, powerful, and has good reason to hold a grudge against most all of the villains.
Smoke: Since I see Noob rising towards 'main villainhood,' someone who has lots of experience with him, and a good motive (say, once being mind controlled by him) to hate him would make a great hero.
Rain: As a very underdeveloped character, Rain has a ton of directions he can go in, especially with the recent war between Edenia and Shao Kahn. Plus, Noob Saibot ruined that war, and, thus, Rain has a reason to hate him.

People I think would really make cool main heroes but will never, ever happen:

Shang Tsung:
The Evil Sorcerer fought alongside Raiden against the Dragon King, right after he was in a battle with Raiden. Perhaps, the combination of Raiden fighting to aid the two evil mages, rather than watch them die and his noble Sacrifice when all else was failing is enough to turn Shang Tsung's mind around. I think it's possible, and would be a dramatic, interesting, and unexpected character jump.