at the moment, i can only think of one that i miss sorely;
SWAT. Swat has been there in the past halos, and has made the game fun for me.
I’d also like to see something like invasion added, which i thought was fun in Reach. They should also add some form of doubles, team snipers, and some single-species game types with only that specie’s weapons allowed and found. (Covenant v Covenant with Covenant weapons, and same for Forerunners and Humans) Of course this means there would have to be levels required (1 for humans, 22 for forerunners, and idk what for Covenant.), but it is possible. There are many other game types that could be added.
That considered, the two i miss most are Swat and Invasion in that order.
They have Flood, which is their version of zombies. Also in addition i’d like to see some larger maps for Big Team Battle, and some maps that don’t have so much cover, like Blood-gulch/Hemorrhage.
What they really need is the infection game type as for the moment you can only have one weapon for the flood, very disappointing for the flood not able to hold gravity hammers.