Good Ideas! A counter-assassination would be different, but if there is one thing I've always liked about the Halo franchise, there was never any 2nd chances. If you're snuck-up-on, your going to get whats coming to you.
I agree with limited matchmaking vehicle's. Even though the limited vehicle weapons make games more challenging, it would be nice to be able to use a Banshee or Falcon (Hornet) to it's full potential.
The A.I. idea you have is something I've been wanting for a long time. Firefight fills that void pretty well, but I have to admit a "Goldeneye-esc" single/multiplayer game on a multiplayer map with your buddies would rule (Slappers Only!).
Different pistols would be sweet. (the pistol in Halo 2 sucked btw) Frag and Stickey grenades are perfect just they way they are, Fire and Spike grenades would be nice to see again. What about a Flash-Bang? Some other members were taliking about a Cryo grenade in a different thread.
I never liked the idea of taking features from other games and applying them to Halo. Halo has always been about a kick ass FPS gameplay experience. Hopefully there wont be a huge change in gameplay. Bungie has done an awesome Job, they've had me hooked for the past ten years.