So I know I didn't mention this, but uh.........vehicles.
Vehicles are WAY more fun when they are not realistic (this could be said about FPS's in general; I'm not here to argue that). I'm not talking about hovercars or alien vehicles either. I'm talking about indestructible weapons of death. One thing about the original Halo that'll always put a smile on my face is the warthog. There was nothing better than setting a trap for the unsuspecting hog rolling my way and watching that bad boy fly way across the level only then to be taking by me. (I also liked the fact that you could remotely blow up grenades using another grenade to perform that very gratifying super explosion)
That **** was hilarious. In Halo 2 and 3, they made them destructible; taking away the "grown man's toy" nature of the warthog. That was a sad day.
Instead, they really should of expanded on the toyish nature of the warthog instead of breaking it down and making the vehicle more "fair". For instance, I would have loved an item that could drive the vehicle remotely. I don't know how it would work; maybe you hop in the driver's seat, attach it, then bail. You'd then pull up the item (a DS!) and then direct the vehicle. Anyway, this one item would open up a wide range of gameplay, and for the more clever, a wide range of ways to brutally kill and humiliate your opponent.
Imagine rigging up an assault vehicle full of grenades and time mines then remotely driving that bad boy into the enemy camp. I can't imagine the look on people's faces when they know what's coming next. However, since the vehicle is susceptible to grenades and rockets, you'll have to be a damn good driver in order to make your delivery of death or else you'll see your car fly sky high.
Now take that very idea and instead think of a boat. I don't know about you but I fuckin' love boats (in real life and in videogames). Every time I'm in a boat I feel like something bad ass is about to happen. Maybe I'm wrong, but I have yet to see a FPS incorporate boat play as well as they incorporated trucks and cars. Now why is that? Is it the level design? Would it be too hard or take to much time? Or maybe it's because graphics would require too much to render usable, swim-able water. Still, I can't imagine how much fun it would be to play in a level where you and your buddy can hop on a jet-ski and have some head to head brawls on open water. Or imagine how satisfying it would be to completely annihilate an opponent whizzing by on a jet-ski with a perfect rocket from across the map. You could even take that further and have ramps, providing the option to use the jet-ski as a bomb. Now, jet-ski's don't have to be the only form of water transportation. You could have basically the equivalent of a warthog, but a boat. It would be more sluggish, but have a higher top speed. The gunner would have to compensate for the waves, but luckily you'd be playing on the Wii.
The possible game designs and level designs are endless.