Season 3 & Beyond: The Human-Covenant War
The next season - and, indeed, potentially quite a few seasons after - could easily draw further inspiration from the Halo novels. Eric Nylund's Halo: First Strike tells an essential story; after the destruction of the first Halo Ring, how did the Master Chief and his allies make it home? At the same time, it also deepened the lore of the Halo universe, revealing that the planet Reach was actually a Forerunner construct in its own right. When the Master Reach returned to Reach, he discovered that a number of his Spartans had actually survived the planet's fall.
There's essentially unlimited potential in this time-period. Nylund's novel expands the cast subtly, meaning the show could then follow multiple side-stories. The plot also introduces two major new characters; Tartarus, leader of the Brutes, and the Elite who would ultimately become the Arbiter. The history of the Forerunners is a subtle plot-thread that continues to run through the story, with the threat of further Halo Rings looming large.
Of course, the plot would then move on to take inspiration from Halo 2 - which kicks off with the Covenant attacking Earth. Soon we're seeing another Halo Ring brought into play, and by the end of that season, all remaining installations have moved to standby. Fortunately, the Master Chief returns to Earth on a stolen Covenant vessel, and the season could end with a classic scene - as the Master Chief explains just what it is he aims to do. "Sir," Spartan-117 reports, "Finishing this fight."
Final Season: Finishing the Fight (And the War)
The Master Chief's return to Earth is detailed in Brian Bendis's tie-in comic Halo: Uprising, and that could easily form the basis for a good few episodes - before we finally launch into the events of Halo 3. The plot of this season would see the Master Chief and the Arbiter join forces against Tartarus and the Prophet of Truth, desperately attempting to prevent the ignition of all the galaxy's Halo Rings - and the extinction of all life. This is the logical culmination of the Halo story, with the Master Chief finally using an actual Halo weapon against the threat of the Flood.
Although the Halo games have continued, it actually makes sense to end the story here. The Human-Covenant War would fits as a single overarching narrative, and there's a beautiful poetry in the Master Chief's use of a Halo Ring to finally bring an end to the bloodshed and war. The series would then end on an Arthurian note, with the Master Chief entering cryogenic sleep, destined to return at the moment he is most needed.
The Halo story is one with almost unlimited potential. This bare-bones story could form the basis for at least five seasons, potentially more, all linked by a single overarching narrative thread. The star of the show would be Spartan-117, but choosing to tell the tale of his origin first would ensure he became an actual character, rather than just a one-man army. As the series continues, the stakes become ever-greater; the threat of the Halo Rings becomes a terrifying, galactic threat.
TV shows like Game of Thrones have proven that it's possible to adapt other mediums into some of the most successful TV series; there's no reason this couldn't be done with Halo.