I'm not normally into reposting, but I was writing this as the announcement was made and it makes more sense here, so...
OK, so I've been thinking of the inevitable story trailer... you know, after which, allegedly, it'll be obvious who the main bad guy of MKX is. I must admit, I am... perplexed.
First, let's talk about what we know. We know for SURE that the main boss is a "fun, surprise returning character" - this rules out Kotal Kahn 100%, and Quan Chi and Shinnok 99%. If it rules out Raiden depends on whether the surprise is that the character is back, or the surprise is that they're the final boss. Ed Boon is no grammarian, so I'm going to leave the door tentatively open for Raiden being the boss. We know for SURE that part of the early story deals with the Netherrealm War, and a chunk of the second act deals with the Outworld Civil War.
Next, let's talk story trailer. For a story trailer, you generally want to give away as little as possible. You want to hype without spoiling. Sometimes that's just not an option. There's no way to hide who the main villain of MK9 is, because the entire plot of the entire game is to reboot MK1-3 so as to kill Shao Kahn. OK, makes sense. In MKX, however, a chunk of the early story involves the NRWar, which isn't directly tied in to the Outworld stuff that comes later. So if I'm NRS's marketing guy and I look at MKX, I'm going to say "let's have 90% of the story trailer be about the Netherrealm War! It'll be huge and dramatic, the stakes are epic... and it doesn't spoil shit past the half hour mark!" As a marketing thing, this makes perfect sense to me.
Except we know that the main bad guy should be obvious from the MKX story trailer. So let's go through the possibilities:
Let's say it's Liu Kang, he's back and he's pissed. Why the hell would they spoil a twist that big? Unless he shows up early enough to fight for Quan Chi during the NRWar... and fights for Quan Chi during the NRWar... it doesn't make any sense. He's not going to have much to do with the Outworld Civil War, so it's not like it's difficult to show off dramatic parts of the story that don't involve him.
Let's say it's Raiden, he goes coocoo for cocoa puffs. Well, Johnny Cage is Raiden's Champion, and Sonya and Raiden get along fine 15 years into the story, so obviously people aren't fighting against Raiden in NRWar or in Outworld Civil War. The only way for him to be obviously the bad guy would be for them to spoil the actual ending of the game, which doesn't make any sense.
And the same is true of most characters I can think of, e.g. Shang Tsung. So who can this magical "fun, returning" character be who makes a big splash either during NRWar or Civil War or both? Maybe I answered my own question with Liu Kang fighting for Quan Chi, but that's a way darker interpretation of the character than I would expect them to go with even if he's the bad guy. If not Liu Kang, I honestly have no clue.
Or maybe the trailer really does give away the whole story needlessly. I guess we'll see soon enough.