I always thought it was implied that the Flood would return someday. I thought blowing up High Charity would've stopped the Flood, but they still showed up to stop you from lighting the ring. The Flood is way too expansive and powerful to be stopped like that. And maybe I'm reading too much into this, but it seems like the UNSC still thinks the Flood is a threat with War Games running a Flood combat simulator.

I'd really like to the see the Flood as a primary antagonist in Halo 5, and this time they wouldn't push over so easily. The Halo Trilogy really downplayed the threat they possessed the threat they possessed, and that's partially why I didn't think they were that interesting to fight.