So, after like 3-4 hours searching information etc, me and my friend got some pretty cool theories about Halo 4, we're still working with them, but yeah.
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1.Didact

Why is Didact evil and how can you be sure he is in game?

Answer: Didact got upset, because master chief destroyed THREE installations. Confirmed by 343 Guilty Spark. Also, his quote:
"I have long dreamed of this day reclaimer"

Edit: That big ball, is actually Didact's ship. In book, Cryptum it mentions, that Didact have ship similar to that.
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2. Precursors

Will Precursors have anything to do with Halo 4?

Answer: Not sure. They are Ancient Evil, in my opinion. There are quote, said by The Timeless One:

"They are about to give it powers it never had before.

Just as they did ages ago....

Those who are ignorant of history are doomed to repeat it."

What does that mean? Also, in Halo 4 concept art video. There is character that is similar to Precursor. Who knows?
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3. 343 Guilty Spark

Will 343 Guilty Spark have anything to do with Halo 4?

Answer: Now, you may say: "Dude, he is dead. You killed him in Halo 3."
Oh really? Did I. During Battle of Installation 04, he did clones of himself, so he may be alive.

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4. The Flood

Will Flood have anything to do with Halo 4?

Answer: 343i said, that we will not FIGHT Flood in Halo 4, but they never said if we will see them or not. Also, I heard tons of people saying, that 343i lately said, that Flood may be in Halo 4, but this is unconfirmed.
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5. Promethean Knights

Are they Ancient Evil? Do they fight everyone?

Answer: I believe, NO. They were too weak. Enough said

Edit: They are Forerunner, The Prometheans are the highest and most respected group of Warrior-Servants in the Forerunner Ecumene.

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6. Cortana

What about Cortana?

Answer: Maybe she is enemy later on? In trailer, she says "You are not, allowed to leave this planet" She is rampant. Chief tries to help her, or kill her.

More things coming soon

When Cortana was doing her short rampant talk, she turned an orange color, like the robot things, and the Didact's symbol at the end. So I was thinking that the Didact could be messing with her to or something... Or maybe they just thought orange looked good for a rampant ai.

It would at least help explain why she wouldn't want you to leave the planet.

Apparently, one clone of Guilty Spark hijacked a human ONI ship in an attempt to find the Librarian, so would be interesting to see if she pops up, if just to stop the Didact.
Also, maybe the Didact isn't evil and just being a prick by using the robots to test the humans and see if they were worthy of the "Mantle" (duty to protect all life) like the Precursors did with the Flood, although that would be a really d**k move.

Humans being in control of all life in the galaxy seems like a bad idea to me.

Okay guys! I edited Didact part! I have more evidences, that Didact is in game. I'm 100% sure about that!

Edit: I also edited Promethian part!

Saying "Guilty Spark cloned himself" is probably the most uncreative way 343i could have gone about bringing him back...

Kusa 'Aezotee wrote:Saying "Guilty Spark cloned himself" is probably the most uncreative way 343i could have gone about bringing him back...

Yeah.

"Hello reclaimer, I cloned myself"