I have seen various warthog concepts in the past. specifically built for snow terrain with different mountings from anti air missile launchers to machine guns... flamethrowers, ect... they should have a whole armory full of different types of warthogs with different kinds of field upgrades.
-A 2 man sniper recon squad sounds nice too, it would go well with a platoon of marines trekking thru the terrain. Allow the ability to have marines sit on a scorpion tanks like in original halo.
vultures were always nice. I don't recall the covenant ever having an equivalent assault ship, im sure a nice one exist in the halo universe tho, this would be a nice addition. keep the ability where air units can fly above high cliffs and mountains staying safe out of grounds units range. I vividly remember even scarabs could climb up onto these tall hills and get away from marines and tanks.