#Precursors, Flood, Janus Key, Iso-Didact & the Librarian are all in Halo 5’s story#
I would’ve written a story that reintroduces the Precursors and has them coming back into the Galaxy as the main antagonist.
It would’ve began with Chief hunting down Jul M’dama to rescue Halsey and recover the Janus key. Chief & Halsey return to the infinity with the complete Janus Key and Lasky orders the infinity to set course for the outer reaches of the Galaxy to locate one of the most intriguing remnants of lost forerunner tech.#
Long & short, this forerunner remnant is a trap set by the Precursors millions of years ago, left there as a beacon notifying them that a civilazation, equal to or more advanced than the forerunners, has taken control of the Milky Way. once the beacon is activated, the Precursors & Flood (note, the precursors come in their original form and we find out exactly how the Precursors created the flood and for what true purpose) immediately launch an all out invasion of the Milky Way from the Galaxy they have been rebuilding their forces in.
This forces the UNSC, Arbiter & remaining covenant forces to form an alliance (with hopes of creating a new Galaxy spanning government) in order to defend themselves against the Precursor Invasion.
During the invasion, the flood completely destroy all remaining Covenant factions except the small packet of Jiralhanae. The precursors understand the asset they have in the brutes, indoctrinate & stimulate their intellectual evolution, making them a terrifying new threat the UNSC must contend with.
During the invasion, Halsey continues to uncover the secrets held within the Janus Key and just as it appears all hope is lost with the Precursors, Flood & Brutes are taking control of the Galaxy, Halsey discovers a way to contact the Librarian & Iso Didiact (who are also in another Galaxy rebuilding their forces). The Iso Didact orders his entire fleet of promethians (actual forerunners) to immediately set course for Eryde-Tyrene (Earth).
The campaign ends as the Arbiters forces, 117, Lasky & Infinity prepare to defend, and if necessary, make their last stand on earth, but just as it appears all hope is lost, the Iso Didacts forces appear from slipspace. thousands of Forerunner dreadnoughts combined with UNSC ships form a picket around Earth and just as the first shots fire between the UNSC/Forerunner force against the incoming Precursor invasion, the screen goes black…