User blog comment:Apollostriumph/Idea for a new extended universe!/@comment-1210220-20160413042038

I should probably note here that due to our Canon Policy, all new universes should follow the current canon as set out by Bungie and 343 Industries. While in the past events taking place around the latter half of the 26th Century (2560+) were more or less open to interpretation, the events of Halo 5 and its galaxy-wide consequences might really interfere with anything beyond 2558. It's actually something the Administration is probably going to decide on in the near-future.

In regards to the Precursors returning, I was under the impression that (from what I could garner from Halopedia at least) that they were the Flood after heavy mutation, which makes their return rather unlikely. Plus there's the fact that they were so powerful that by the 26th Century nobody could realistically fight them on open ground. My advice to anyone on the site wanting to create an Expanded Universe would be to at least have a base of sorts around the recent Halo timeline of the 2550s to set up characters and features for later on. If you just want the Flood to come back around this time, then it's entirely plausible to do so - perhaps after taking over High Charity, Gravemind sent spores to some far-off planet? I hope this helps.