People also complained when Pure Forms were introduced to add that same variety to the Flood.

I think that Flood-only levels are bad. Ex. The Library, Cortana, etc.

I think levels with Flood, Covenant, and Sentinels are amazing. Ex. Two Betrayals, The Maw.

I would love to see Prometheans(since we know they aren't going away...) fighting Flood and the Covenant in a big battle while Chief and Blue Team cut their way through the carnage. Or Locke and Red Team(for lack of a better term.) I really love the dynamic and it makes the world feel more alive to come into a room and see other factions engaged in combat without you engaging it.