Ok boys and girls here it goes my well thought out story line.
So we now that their floating towards this planet.
I believe that this planet is a forerunner planet that they created after they left the rings they where to use it as a way to rebuild their civilization away from anything that could harm them. This planet would work similarly to the rings as they can create their own oxygen, water, weather systems and everything else that make a liveable planets a liveable planet.
As the flood is hiding (or whatever the hell they where doing when the forerunner left the rings) as they leave to the rings on their ships the flood sneak on board. They stay on the ship till they get to the planet then they infect the forerunner.
Unlike the flood we know of this flood will be intelligent (as they took theform of the forerunners) they would be more like elites then flood (maybe even smarter).
As they land on this planet (cortana and chief) they find thenslves surrounded by infected forerunner they fight their way through the infected till they find normal forerunners still alive. As their is no UNSC or Covenant their is no covenant or UnSC weapon witch lets 343 industries get creative with the weapons, grenade etc.
This could totally refresh the franchise and start a new trilogy.
Chief and Cortana objective is to eradicate the flood and get of the planet.
I know that 343 industries wants to make it a trilogy so i haven't thought of and ending.
i kept it brief because it's not up to me to make the damn game if u want me to expand get 343 to hire me. And people can turn it into a billion different things, this way i just left base point so nobody can interpet it differently.
hope u engoyed