I don't know if you've heard, but 343 studios already has three future Halo game ideas projected to be made by 2020. I can't confirm it yet but one of my friends at Bungie told me about these ideas last year.
The first idea is for a Halo Wars 2. The new game will include new battles, maps(Reach included), as well as a "Wars" style Forge Mode.
The next is an alternate storyline of the original trilogy called Halo: The Covenant Chronicles. It follows the the 3-part saga of the original story, but from the Arbiter's side of the war. You'll see his rise to leadership, his rebellion against the Prophets, and ultimately his victory beside the Master Chief.
The last idea I heard about should answer your question more directly. The initial title is Halo: Forerunner. It's supposed to be the next installment in the Master Chief/Cortana story.
The story takes place thirteen years after the destruction of the Arc in Halo 3. The human race and Covenant have returned to their home worlds to rebuild and John-117's life pod is still drifting out into deep space. News of the Arc and Halo's destruction has reached the Forerunners who have returned to investigate. On their way back to Halo, the Forerunners find Master Chief's pod. They learn what they can about the war from Cortana and decide to upgrade her and Master Chief for a new mission.
Back home, the Human and Covenant planets are invaded by a new enemy; The Flood Queen. With the Arc and Halo rings gone, nothing can stop the Queen's invasion as she breeds new Graveminds to conquer the galaxy.
With their new Forerunner upgrades, Master Chief and Cortana must now travel back in time to change history. Their new mission is to save the Halo rings and the Arc without interfering with the objectives of their past selves.
The new story suggested new gameplay (such as using Cortana as as a co-op character), new Forerunner weaponry, and new light-based abilities such as teleport.
I wish I could say this was all definite in 343's plan. Unfortunately, for legal reasons I can't even confirm my sources. Still, I can say that this information is NOT a rumor. Whether these concepts become games or not, it is a fact that Master Chief's story will continue and other Halo projects ARE on the way.