Halo Reach: oh noez the Covenant! worst thing we could ever face!
Halo CE: whew we just barely escaped TFoR. O.O a new mysterious world to explorz! o noez we were wrong! the covenant arent the worst thing we could ever face! its the flood!
Halo 2: whew. the flood is gone for good. oh noez! we were wrong! theres more flood!
Halo 3: ok. NOW the flood is gone. worst thing humanity could face, but gone for good this time. plus, no more covenant! 
Halo 4: whew… we just barely escaped the battle of earth. O.O a new mysterious world to explorz! o noez! we were wrong! the flood isnt the worst thing we could face! its ____!!
anyone else beginning to see a pattern??
my guess is Halo 5 and 6 will turn out something like this:
Halo 5: whew. ____ is gone. o noez! we were wong! theres more/worse ____!!
Halo 6: ok. NOW ____ is gone for good.
???