For Halo 5 and 6 I think the plot should go like this:
MC is sent with a new Cortana replacement AI to investigate a science team's mysterious disappearance on a newly discovered Forerunner planet. When he arrives he finds the Didact has survived and after a fight the Didact is about to kill MC when a Forerunner AI informs him that a huge Precursor fleet has arrived in the galaxy seeking revenge now that the Flood have been wiped out. The Didact realizes that he alone cannot fight the Precursors so he recognizes MC as a new ally, understanding MC is skilled enough to have defeated him once. The Forerunners, the UNSC, and the Covenant have to work together to defeat the far technologically superior Precursors. This would be when we met up with the Arbiter again, because everyone including me wants him to return. This would be the basis of Halo 5 and 6, but of course the details would be worked out by 343i.
As for the return of the Flood, I would rather fight the actual Precursors than their creations. I want the Precursors to have a feeling of vast technological superiority like that of the Forerunner Prometheans instead of that of Flood.
