I don't know why I posted it in about 3-5 different topics before making my own, but here's some things you should know.
You will not be fighting Forerunners or Precursors. Both are considered extinct. Forerunners have confirmed the death of Precursors as no longer existing, they were believed to have been the Forerunners creators. The evidence supporting Forerunners death is simple because they would not allow the Covenant to go around untouched on their planets, and Halos. There has been no sign of them since the activation of the rings, and believe me, if there were any left around, you would have seen them.
It's debatable that humans are related to Forerunners considering they had existed before humans, but it's been said that we are descendants of Forerunners. The Forerunners chose the human race as part of their re-population of the universe if the Halo rings were to be activated, hence why a portal to the Ark was place on our planet. The Forerunners were the guardians of the galaxy and soon moved beyond the belief of needing weaponry, which leads to their downfall. They had the power to stunt evolution or increase growth of intelligent life. An example would be us, humans they performed devolution of the human empire to prevent further conflict emanating from what they considered a "violent species."
An Ancient evil does not refer to Precursors or Forerunners. Precursors really were nothing to us besides a more advanced race, nothing evil there. Forerunners might as well be considered our guardians, they deemed us "special" in their eyes, they never were our enemy. 343 Guilty Spark and 2401 Penitent Tangent referred to 117, and humans, as "Reclaimers." Like stated earlier, we were their chosen race.
The ancient evil can pretty much refer to only 1 thing, and that is the Flood. What led to the fall of the Forerunners, was an AI defecting to the Flood named 05-032 Mendicant Bias who was turned by a Gravemind on the belief that the Forerunners were stunting universal growth of a superior race. Offensive Bias was created to counter Mendicant. No evidence is available to declare Mendicant Bias was destroyed. Being an AI of Forerunner technology, do not be surprised if this game turns into Sierra-117 fighting off the Flood with Forerunner technology on his side, such as Sentinels.
I foresee this being Offensive Bias on your side, you being a Reclaimer fending off Mendicant and advanced Flood with Sentinels on your side as well as against. There will probably be new weapons, that of Forerunner technology.
Most of this information, and a lot more can be found at http://halo.wikia.com/wiki/Main_Page. Educate yourselves if you are bored. There's a lot of information than you know on this game and it's all actually quite interesting. I really do help this clears things up and sheds some light on what is coming up in the next game.