I think it'll either be a complete surprise and we'll all be like: OMG or it'll be a disappointing choice. I don't see much middle ground. Why? Well, let's talk about it for a minute:
It was said that it's a "surprise returning character" - but that was the context of this answer? I don't remember, but if it was said to some game website or anything of that kind, how seriously can we take this answer? For a casual player, for instance, anyone would be a surprise returning character. Shang Tsung, Liu Kang, Sindel, heck, even Jax would be a "surprise returning character" for someone who isn't that much into the game. A casual player wouldn't expect Jax's giant boot to be the boss.
Then we see the other side of the coin which is the hardcore MK player... what would be a true surprise for such player?
Liu Kang? But we already have the feeling that it may be him and there were hints towards this before. You take his MK9 ending, the fact that he's not announced yet and his previous storyline (he dies, becomes a zombie, comes back from death to fight) and it is quite a clear possibility. Fire-God Liu Kang is just a new twist for the zombie Liu Kang from MKD.
Evil Raiden? Same as Liu Kang, how exactly is this surprising for a hardcore fan? His previous storyline also had shades of corrupted Raiden, he was called a mad man quite a few times during MK9 and there's this thing called MK Legacy which also shows a crazy Raiden. It's not surprising.
The fact is: throw any major name as an option and we won't be surprised since we have lots and lots of reasons to point such characters as a possible boss. Shinnok, Mileena, etc are all characters that we already saw and more than that: already were thrown as possible bosses, almost as if to distract us. To make my first point clear, let me create three new theories:
Noob Saibot or anything related to him - He was thrown in the Soulnado and his definitive fate is unknown. Sub-Zero's true identity is almost being kept as a secret for MKX. In the trailer we see the new faces looking for Sub-Zero's help/advice/whatever. Then something that resembles a "blood Soulnado" appears as the "blood demon". Mix all these facts in a cup and there's the possibility that Noob Saibot actually got more friggin' powerful thanks to the Soulnado for some reason and is now a huge threat, thus why his brother's assistance is seek.
A new take on Ermac and Liu Kang's spirit - This is a quick theory. In the old timeline Liu Kang's spirit was helped by Ermac. What if in this timeline he actually joined Ermac and the result is some sort of Fire-God-Ermac boss?
Shang Tsung and a new take on the Deadly Alliance - Another quick one. The Deadly Alliance was a huge event in the old timeline. Shang Tsung was the very first MK boss and as the "Goro Lives" teaser shows us, they like to give some nods to the classic games. Somehow he is reborn as the "blood demon" (we're calling it that but we don't even know for sure what it is) and Shinnok being imprisoned in the amulet has something to do with it. A new alliance is born.
But back to my main point, which is the true surprise or huge disappointment... really, who's left to fit the surprise returning character position? You can throw any name there and most of them won't satisfy. I just created three new theories which could be "surprising", but would them make fans happy? This is my point: in the end it all amounts to possibilities we're aware of and are not surprising or possibilities that could be surprising but aren't much satisfactory. It'll either be a HUGE surprise (and I don't see how it can be) or a disappointment. Be ready.