So they’ll probably have the standard achievements. Beat each level, beat the game on all difficulties, probably some for the terminals. Aside from that kind of stuff what do you want?
Personally I feel they should have some achievements for finding the thirst grunt, maybe one for switching between the graphical settings and finding “The Siege of Madrigal” track in the “Assault on the Control Room” level.
An Achievement along the lines of “Sorry, no sword for you” would be funny for when you kill you first sword wielding Elite referencing how you couldn’t use the sword in Combat Evolved. Something like “Over Powered” or “Hand Canon” would be a nice achievement name for killing a hunter with a single pistol shot to the back.
“Keep your friends close and your shotguns closer” would be funny for completing the Library in co-op. “Two Spartans, One Achievement” would be a funny achievement for beating the campaign in co-op. An achievement called “Why it’s rated M” for when you first encounter the flood would be a reference to claims of the flood being the main reason the original Halo was rated M.