Could the Flood be making a return in the next-gen Halo? I certainly hope so; I miss the horror/thriller aspect they brought to the series. If the Flood do make their return, 343 should keep it a secret and take a Halo: Combat Evolved approach. Of course, they should have new abilities. Maybe two Combat forms could combine into one giant one (just throwin’ out ideas).
On the other hand, the Promethean A.I should be revamped; they didn’t feel too different from the Covenant in Halo 4.