Think about it, the Covenant are returning (maybe not all of them), the Flood will almost certainly be there as well and of course the new enemies. The potential for 4 way fights with very different AI is amazing, Chief and the UNSC vs Covenant vs The Flood vs the new enemies, all fighting against each other. One of the best things about the first Halo was the introduction of the Flood, and the fact that they would attack you and the Covenant. This is mostly speculation on my part regarding the Flood, but we do know there is a new enemy and that the Covenant will return.