We see these crawlers, watchers and Promethean knights in the E3 Trailer. Cortana says upon engaging the crawlers that they are Defense AIs and definitely NOT Covenant. She also recognizes the design of the weapons and says it’s Forerunner.
So I conclude the enemies are Forerunner machines. And in each machine there is a Forerunner AI, also called Ancilla. But it could also be that these AIs are actually imprinted Forerunner personalities. The Forerunners were able to imprint their personalities into their machines such as the Monitors. They did that with humans like CHAKAS who later became 343 GUILTY SPARK. But also the Forerunners were able to do so: the Lifeworker Genemender Folder of Fortune had his own personality extracted from his body.
In the Trailer, a huge ball-like thing appears behind the Master Chief after he watches he UNSC Infinity descend. That huge ball scanned the Master Chief and flew away. In the concept art it is appears as a CRYPTUM. Here is the link:
http://www.halopedian.com/Cryptum
Now last point: We hear a very iconic voice at the end of the trailer, saying “I HAVE LONG DREAMED OF THIS DAY, RECLAIMER.” Then a symbol appears and is turning upside down. Albeit red, that symbol DEFINITELY belongs to the DIDACT. Here you can see the blue version:
http://www.halopedian.com/The_Didact
CONCLUSION:
I think that the Didact is INSIDE that Cryptum which scanned the Master Chief. I also believe that the voice we hear at the end of the E3 Trailer belongs to the Didact. It would perfectly fit him in my opinion. Also there is a soundtrack in the Trailer that can be heard when Cortana talks about her having passed the deterioration date of normals AIs. It is like “SA HA HA HA HE HI HU HE”. I think that his woul be a perfect theme for the Didact and somehow matches his voice. This is all assumption, of course.
Furthermore, I believe that the Didact has plans with the humans. He says “I have long dreamed of this day, RECLAIMER.” “Reclaimer” – a term he wouldn’t use without purpose. Throughout the original Halo series, we discovered that the humans were the reclaimers, inheritors to all the Forerunners left behind. What if there was a purpose for that? The Didact wanted them in particular to reclaim Forerunner technology. And they are the only ones who obviously can reclaim the Forerunner technology as we see in Halo 2 and Halo Wars: In Halo 2, TARTARUS needs Miranda Keyes to activate Delta Halo’s firing impulse. In Halo Wars, the ARBITER needs Professor Anderson to activate the Forerunner Ships.
I also wondered why the Forerunners catalogued ALL sentient lifeforms in the galaxy BUT NOT THEMSELVES before activating the Halos. So here is my hypothesis:
The Didact wants the Forerunners to ressurect. He put their personalities into the machines and now wants to imprint them in the bodies of the humans. The humans reclaim the lost identity of the Forerunners which are the Ancient Evil rising now.