I'm gonna say it's Chameleon-just a hunch. Would be pretty sweet, anyway.

No, it's striker

Unlocks all the DLC.  If you already bought it, Shao Kahn laughs at you.

I don't think it'd be an unlockable character. Probably like a special video?

" I don't think it'd be an unlockable character. Probably like a special video? "

I dont know, Ed and his team are hyping it up a lot. Maybe it will be MK 1, 2, 3 HD ?

It actually replaces all characters in the character select screen with Quan Chi.

I dunno, probably just unlocks all the hidden stuff.  Or maybe it unlocks documentary of the making of the game.   I dunno, MK guys have a strange sense of humor.  It might not be anything but the coins and maybe Ed Boon congratulating you. 

" I don't think it'd be an unlockable character. Probably like a special video? "

A picture of Ed Boon.
 
I don't know, I'll let you know when I get the game.

it could be alternate costume(s)

Playable Shao Kahn. 

French Stewart comes to your house and sings happy birthday.

The challenge is you have to make John Romero your bitch!

You unlock Dan Forden

You unlock Tao Feng: Fist of the Lotus courtesy of John Tobias.

" The challenge is you have to make John Romero your bitch! "

You unlock Rain, and he's a giant stop motion sprite.

" It actually replaces all characters in the character select screen with Quan Chi. "

Batman. 
 
Kombatman.

If it's anything as remotely awesome as the Hand Cannon in Dead Space 2, I'll be happy.

unlocks the toasty dude.

"I dont know, Ed and his team are hyping it up a lot. Maybe it will be MK 1, 2, 3 HD ? "

Jackie Chan comes up and says that this is his game now, and then you will have to control every character in the game at the same time and fight Jackie Chan and his fists of fire, he also has three forms. 
If you beat him, you unlock him. If you lose, the MK part of the disc will be removed and replaced with "Jackie Chan in Fists of Fire", forcing you to buy a new copy of the game if you want to play more MK.

" Jackie Chan comes up and says that this is his game now, and then you will have to control every character in the game at the same time and fight Jackie Chan and his fists of fire, he also has three forms. If you beat him, you unlock him. If you lose, the MK part of the disc will be removed and replaced with "Jackie Chan in Fists of Fire", forcing you to buy a new copy of the game if you want to play more MK. "

An achievement. 

" I dont know, Ed and his team are hyping it up a lot. Maybe it will be MK 1, 2, 3 HD ? "

Jeff Gerstmann.

"Jeff Gerstmann. "

The reward is...

Ed Boon gave an interview where he referred to the reward as something someone could see in an online match.  So I'm guessing it's got to be a skin, or maybe a change to your online name or something like that.

" @xhavoc86: That sounds way too underwhelming for how much they're hyping it up. I mean the achievement list itself had something cool as Cyber Sub-Zero, which is infinitely more cooler than that. Not saying it's BS, but I kind of hope it is. It comes off a bit tacky as well. "

I think an alternate costume like that would have shown up in the banning report, wouldn't it? On top of that, Tower Master is the achievement you get for finishing the Challenge Tower, so why would there be another Achievement tied to it? At the very least that seems to indicate that the reward is something else besides that alternate.
 
At any rate, I question why people are mining the demo stuff anyway. Aren't demos just special builds of the game (in this case one that's 8 weeks old)? Why would they compress the entire game into the demo and then send it out? Unless the demo is gigs of data large, that should send up some red flags; I mean, ifI was a dude working on Mortal Kombat, I would know that people were going to data mine and would definitely put a ton a red herrings into it to mess with people. But who knows?

" I think an alternate costume like that would have shown up in the banning report, wouldn't it? 

Maybe it unlocks a special finisher that any character can perform.

Unlocks Nudalities.

I think you get the naked Mileen alt...

It's a loooooooooong shot but my guess is it unlocks friendships, animalities, babalities and brutalities 

" Unlocks all the DLC.  If you already bought it, Shao Kahn laughs at you. "

Mortal Kombat theme music from the movie.

A big static screen featuring all the MK Kharacters up to various hijinks and a banner that says: "CONGRATULATIONS!!" hanging above them.

Ed Boon will come to your house and replace all your C's with K's.

I hope it's HD versions of MK 1 2 and 3.

" A big static screen featuring all the MK Kharacters up to various hijinks and a banner that says: "CONGRATULATIONS!!" hanging above them. "

Third Fatality for everyone. They already designed a third one for the ninjas, so why not just give it to everyone else and make it crazy-difficult to unlock?