Yes, i have seen this as well, concept art and pictures of the ball have it labeled a cryptum. unless we are bringing the master builder into play here, which may prove unlikely because he has not been mentioned in any of the games, only in the books, then the most likely "ancient enemy" is the didact. the didact, even though he may have been on better terms with Chakas and other humans after they were defeated and de-evolved, was the forerunner leader who destroyed the human/san-shyuum empire. on halopedian, it says that "The Didact, commander-in-chief of the Forerunner military, saw humanity as one of the most contentious, bigoted, and self-centered species in the galaxy." that little tidbit was pulled from Cryptum if I am not mistaken.
I would be thrilled if we had the Didact as a friendly character. I think he would respect the chief, and his tenacity and sacrifice in fighting the "stinking menace" of the flood. I also think that the Didact, in his normal reasonable state of mind, even with his temper, would not seek to destroy humans. Where is his motivation? the current humans have done nothing to earn his ire. sure, destroying the lesser ark might be a big deal, but I'm sure the Didact would have done the same, if it meant the removal of a gravemind. It was necessary. there may be something we do not know about, as I am sure that the Didact must know something about the discovery and use of onyx/Trevelyan by humans, and our use of forerunner tech to build the Infinity. so unless he is just really pissed about humans using forerunner tech and worlds to further their own goals, there are not a a lot, if any, strong reasons for the Didact to wage war on present day humans.
But, we must also factor in Greg Bear's cryptic quote about two Didacts. In the trailer two different colors can be seen in the Promethean constructs on requiem. there are blue, and there are orange/reddish. though unconfirmed, this could signify two different factions of promethean on requiem. who might the leaders of these factions be? is it one Didact, though he may have some sort of damage to his mind (100,000 years of guilt over using the Halo array and thinking he killed his wife?), who later on is redeemed/restored to sanity/normalcy to become an ally, and bring some of his promethean servants with him over to the chiefs side? or are there two literal didacts, one an ally/neutral party, and one crazed, vengeful incarnation of humanities great destroyer during the human-forerunner war? remember, in Bear's novels, the Didact expressed regret over his near annihilation of humans, as he did not know that the cause of human expansion was the flood.
To sum it up, i think that the Didact, in his normal state of mind, as he was at the end of primordium, would not try and destroy humans. it is possible that he has gone a bit crazy and has some sort of split personality, or there is another manifestation of his consciousness or memories in another cryptum or forerunner body/construct. the turning of his glyph upside down has got to signify that something has gone wrong with him, that some issue exists that is impairing his judgement or somehow he has been changed. his rage and fight against humans during their war definitely classifies him as an ancient and terrible evil, to humanity during this period in time (human-forerunner war).
i bet greg bears third forerunner novel sheds light on what happens to the Didact, and it is no coincidence that it is not coming out until after Halo 4. again, speculation, but i bet something happens in that novel to change the Didact, thus explaining his hostility/brokeness/the fact that there are two of him.
Oh and MB probably wont be back. I am near 100% certain he went down with the lesser ark. OB, or some other metrarch class could make an appearance, but after the array fired, it is likely that the forerunners didnt create any more like MB or OB because of MB's rebellion.
And dont forget the loss of the domain-losing the entire history/collective memory of your civilization can probably have big effects on any forerunners left alive after the array fired.
