Okay, first off. It's a wiki, yes I know you read that, but let's be honest, it's the first week the game has been in circulation. Let's face it, no one knows what the case is until we see The Didact reappear. Another thing is that anyone reading that page should see is what it says about his deaths. "Second Body". Refering to that his soul/fragmented reality can be recreated/reimerged into a new form. Even if he did die, I believe we have not seen the last of The Didact.
As for Cortana, I really hope there might be some strands of her left on the chip, or in The Composer. I believe we will be seeing Halsey again in result of this.
Another thing also to mention is that in the Legendary Ending, you see Chief's armor removed. You see his face. I presume that since that since his armor is damaged too the extent of high disrepair, he will be outfitted with a Mark VII suit, or a new Mk. VI. Whichever one that the Spartan IVs wear. Possibly a new AI.
Another thing will be the possible return of The Arbiter. We shall see how this progresses.
More importantly, we shall see what happens in Nevada/South-West USA. Possibly a Second Battle Of Earth? Who knows!
2 More years men, let's see how this develops!