Post the worst but canonically plausible Halo 5: Guardians Campaign stories and theories in here.
Here's one I pulled from a member on HaloArchive named Vegerot (who actually found it somewhere himself):
The Flood have conquered the entire galaxy. Everyone has been consumed. The Master Chief, the Arbiter, everyone: gone. Only a small group of Prophets remain. They discover that for thousands of years, their home world of Janjur Qom is truly still intact, as was theorized in the Bestarium. The Reformist San 'Shyuum population has gone unnoticed by the Flood. Now, it is up to the Prophets to raise an army by increasing the entire population. To increase their numbers, the Prophets must GET IT ON. Basically, Halo becomes a gross porno/romantic comedy until the final, epic fight with the Flood.
Here's one of mine:
We follow Chief and Locke on what seem to be paths that eventually intersect only to be surprised that they are not even in the same galaxy however Locke does come across Chief... His flash clone all grown up living in a stolen Mjolnir suit and he's with the first Spartan IV from Initiation who is now his wife. Locke does hunt Chief but not the original Master Chief and kills the clone and the wife gets away again. Meanwhile, original Chief and Blue Team are fighting a Guardian that's guarding a burial chamber under Sangheilios that can only be opened by Chief because of his geas in order to get the last fragment of Cortana that was pulled from the Domain. Arbiter joins him and Kelly gets killed in a Kat/Carmine fashion where a bullet from far away shoots her in the head. The shooter is The Ur-Didact and he's now a Promethean Crawler because they were out of parts to make anything else for him but at least he's one of those spiky crawlers and not a regular one. Jul Mdama and Halsey are watching overhead from orbit and Halsey goes crazy after seeing Kelly get disintegrated and she starts glassing Sangheilios. Chief and the rest of Blue Team along with the Arbiter must get on a Warthog and escape the exploding planet while it is being glassed. In the end, Chief and the team make the jump but only half the ship they are on makes the jump into slipspace. The last thing we see is Chief in the light of another mysterious planet.