True. I feel as if the Prometheans had the potential to be scary though, but it seemed obvious that it wasn't what 343i were going for by introducing them. Unfortunately.
And yeah. There's a lot of potential there for the Flood... If 343i are willing of course. Bungie made them too weak in H3 and coupled with the bright art style, they lost most of the creepy factor that they once had.
I think melee damage made sense, but overall they felt so weak, especially since they practically explode into dust when pummeled. I'd dial back the effectiveness of melee to make them getting in close more threatening. If we encountered them on another ring I'd live for them to infect local fauna as well. Giving us something besides bipeds to react to.
Alternatively, we've got the Flood forms from Halo Wars that could be played with.