Vehicles

ATV
UNSC has no answer to the Ghost. Long ago, when Halo 2 was only a distant dream, and fan boy chatter and questionable screenshots were all we had to go on, the ATV was a sure shot to be a new vehicle in Halo 2. Now November 9th, 2004 has come and gone, and no such vehicle has come to pass. The ATV would make a great flag-running vehicle. I'm thinking something with the agility, speed and size of a Ghost but with Warthog handling and stability.
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APC
I've been playing a lot of Mercenaries these days. In Mercenaries, I love grabbing a bunch of dudes, loaded them into an armored personnel carrier and taking them to a fray. I 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. It could be good stuff. This would be a great addition to multiplayer too.
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Driving a Pelican
Picture it: Capture the flag in Coagulation. You and five of your buddies are en route to the base on the other side of the map. Rockets hiss by, and the ship rocks back and forth as it takes one on the starboard side. As the Pelican banks left and swings wide to avoid another salvo of tracked rockets, you see the enemy base. Two rooftop snipers pour it on, filling the inside of the Pelican with white streaks. Your rocket guy drops behind you. He never even saw it coming. The smoking Pelican belly flops onto the top of your opponent's base in a hail of crisscrossing fire. You and your pals pile out. The pilot is picked off as he exits the Pelican. A flash, and your shields are dropped. You fire furiously to your left and right as you seek cover near the smoldering Pelican. You eye a Ghost outside the base, as you your dive down onto the flag. You mutter into your headset, "That's how one of us is getting out of here."
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Yeah, so the Pelican would be a great edition to multiplayer. And while I'm making wishes, I would say that you should be able stow extra weapons on the Pelican. On large multiplayer maps, the Pelican could function as a mobile base. There could even be a capture the flag variant where the flags are kept on Pelicans.

Boats
What if you could clear that beach in Outskirts with a good old-fashioned Marine beach landing? This early Bungie concept art shows that they at least considered it.
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Some water combat would be a nice addition to both multiplayer and campaign. Imagine a multiplayer map that was a series of small islands. The bases would be on different islands, and it would take coordinated boat attacks, beach landings and sniper fire to get things done. That would be sweet. I love this idea.