Okay, so, there are a lot of mysteries surrounding the new game Young Justice: Legacy. Here are my theories about this.
Mystery Villain: Now, I think that this "mystery villain" is Mordru. I believe so because of that scene where Nightwing says "We won't let you destroy the world". You probably think that he was referring to Klarion, but then why the sudden cut? They could have put "We won't let you destroy the world, Klarion" but they didn't. And the sudden scene cut afterwards makes me think that he said the mystery villain's name. And we saw Klarion in the scene, right? So who could be working with Klarion? Well, probably none other than the Lord of Lords of Chaos himself! But they DO have to think of a way that Mordru could tie his spirit to the Earthly realm. Nabu has his helmet, Klarion has Teekl, so what about Mordru? Maybe he must inhabit the bodies of evildoers?
One will Rise: Honestly, this is the only one that I have no idea about. All I know is that I think we're talking about Artemis here. I think that because they show a picture of Artemis when they say "One will Rise" in the full trailer. And the other two make perfect sense.
One will Fall: This is obviously Aqualad. Of course, he didn't really "fall", because he is on a deep cover mission for what I nickname "the Dark". But the characters in the game believe that this was an actual betrayal. So, I think it counts.
One will Die: Okay, people, when we talk about Legacy's Robin, are we sure that we're talking about Tim? Because after they said "One will Die", I saw a flash of red, yellow, and black that everyone knows is Robin. So this brings to my attention that the Robin of the game is none other than Jason Todd. We all know he died. But we don't know when. At the same time, we think that Tula dies. I think that that doesn't count, because she isn't a playable character, not even DLC. I agree that she will die, though, because of my "One will Fall" theory.
Anyway, those are my theories, tell me what you agree with!
EDIT: So, I have a few new theories. I just checked out the Legacy website, and it talks about Tim as Robin. I still think that Jason will die, but Tim will "rise" by taking up Jason's mantle. Tim will have the same attacks as Jason, and the same amount of levels that you gave Jason, so people won't lose their favorite character. This also gaves me a new "One will Rise" theory, about Tim becoming Robin.