I think the music and sound design should be overhauled to give them back their creepy and tense moments of gameplay. From the music to the screams and attack sounds they make, the Flood should return to the series as something to be feared like in the original trilogy.
Take this track for instance(The Suffering Theme Song)
https://youtu.be/TwlfTjuhkJw
The music should not be orchestrated traditionally and should sound otherworldly. The Flood should be so menacing that the Promethians should stop attacking you and focus on them because of the danger they are.
Anyways, I think their return would greatly benefit the story and campaign by mixing the gunplay from traditional cover tactics that Covy and Forerunner attackers do to the overwhelming force the Flood are known for.
As long as I’m not trapped in a tiny corridor with one shotgun shot, I’m cool with that 