UNSC has no answer to the Ghost. Long ago, when "Halo 2" was only a distant dream, and fan boy chatter and questionable screen shots were all we had to go on, the ATV was a sure shot to be a new vehicle in "Halo 2." November 9th, 2004 came and went with no such vehicle. The ATV would make a great flag-running vehicle. Something with the agility, speed and size of a Ghost but with Warthog handling and stability sounds right.
In "Mercenaries," you can grab a bunch of dudes, load them into an armored personnel carrier and take them to a fray. We wish "Halo" had some kind of APC. Think about how cool it would be to roll onto the beach in Outskirts with an APC full of Marines. They pile out shooting, and combat hi-jinx ensues. This would be a great addition to multiplayer too.