I believe that the enemy of Halo 4 is the Master Builder and his followers.
Reasons:
1)The Master Builder (Faber) feels betrayed by everyone:
Most Forerunners and the council for stipping him off his rank and bringing him to a trial.
And the Didact for supporting the idea of the shield worlds.
2) Faber conciders all the other races inferior to the Forerunners. This means that he would not like the Humans to reclaim Forerunner technology.
3)Perfect chance: He was thought dead on Installation 04. He still has supporters. He created the Halo Array. So this means that if somebody new how to escape the blast of the rings it would be him (secret escape covered by him considered dead).
4) 100.000 thousand years have passed. That is enough time for he to create a new army.
5) The Didact was a Promethean. So who has built Requiem and the other shield worlds?
Probably the Builders. Probably the Master Builder accepted the Didact’s idea, so that he could escape the firing of the Array and keep vehicles and weapons away from the flood so that once he has built an army he could use them to rebuild the forerunner empire and annihilate all those that betrayed him or the ones that he hates (= everyone).
NOTE: As far as the other races of the new enemy are concerned (sorry but if I post their name it will be a leak) they could be enslaved races or created races, like the Huragok (engineers).
Does anyone have a similar or another theory about the ancient evil?