The Covenant can't be the enemy because they're not an ancient threat and they had many casualties,their high Prophets are dead,and the Elites left the Covenant.It can't be the Rebels because they're not ancient.
That leaves the Sentinels and the Flood.Let me start with the Flood.The Flood are an ancient threat and it's likely that they're still alive and Gravemind looked like he would have been a fun boss to face.It also ays something about Master Chief and his destiny and stuff like that so it might be his destiny to kill the Flood and Gravemind because Gravemind lied to him and the Arbiter in Halo 2 and he also took Cortana and Gravemind also talked to Master Chief in some thing in Halo 3 and Master Chief never fought Gravemind which would have been epic.But it's highly unlikely that it's Gravemind because there's going to be a Halo 5 and 6 I believe and it'd be pretty hard to make 3 more Halo games between Master Chief and Gravemind unless there's a new enemy in each of the next three Halo games.The Halo 4 thing said that it's an ancient threat to the whole Universe and as far as I'm concerned,the Flood is only a threat to the Galaxy,not the Universe.
The next one is the Sentinels and if you've played the Halo series than you should know that Master Chief and the Sentinals don't get along and the Sentinals are ancient and a threat to Master Chief.What brings the Sentinals down to being extremely unlikely is that they're only a threat to Master Chief and his allies,not the Galaxy nor Universe.
So it's most likely something new to the Halo series because Halo 4 is said to start a new trilogy in the Halo series,there's two more Halo games after Halo 4,and it says that the enemy in Halo 4 is a threat to the Universe and the biggest threat in Halo that I know of are the Flood and the Halo rings which both are a threat to only a Galaxy.