1. Halo 4
Halo 4 is the start of a new Halo trilogy, and is set after the events of the third game in the previous trilogy. Taking the reins from Bungie, 343 Studios is developing Halo 4 from the ground up.
The war with the Covenant is over and the Master Chief must find his way back to known space after being launched into deep space to counter an all new threat—one that happens to be an 'ancient evil' in the game's terms. Presumably, Master Chief is going to have to deal with the Precursors, a powerful alien race that ruled the galaxy before the Forerunners developed the Halo Array.
Sci-fi readers and fans of the Halo games will have plenty to look forward to in this next iteration of the series.