I understand why they weren't in Halo 5 and it makes sense that they weren't. It probably would have felt a little too early, and there was still more that needed to be explained and gotten out of the way. When I speak about the Flood coming back I always do so firstly because they were always my favorite enemy to fight. With not many people enjoying the Soldiers and the Prometheans as a whole, Enemy variety needs to increase.
That's why I also would like, at the very least, Drones to Return. They had a unique style to them (Flying, Swarming) that added another element to gameplay.
There's probably a handful of people who would prefer that they did do that.