How can you move in Halo? You can jog forward, backward, left or right on the X and Y axis. Jump on the Z axis. You can combine the two to do a parabola shaped jump if you jog first then jump. You can crouch on the Z axis. You can combine crouch with the parabola jump to increase the distance you can jump.
Each element of movement interacts with each other. Each capability, running, jumping, crouching, has its own purpose but can also be combined for different effects.
What additional movement capabilities would add diversity to the game on their own but also be able to interact with the existing movement capabilities?
Sprinting in Reach interacts with the X, Y and Z axis in different ways. At the most basic function it allows increased speed in the X and Y axis. Instead of jogging, jumping then crouching to achieve the farthest horizontal jump, like in previous Halo titles, the introduction of Sprint allowed you to Sprint, jump then crouch to achieve an even further jump. It functions on its own but also provides new effects when combined with jumping.
My idea for an additional movement capability is the double jump. By tapping the jump button again while you are in the air you gain more height. This would help bring some vertical mobility into your movement. A permanent high jump would not work because there are many instances when you do not want to expose yourself for the duration of the high, floaty jump and would prefer a lower jump. A permanent high jump would also not work because it is too predictable. Once you learn the single jump height, which never changes, you can follow a target very easily even if he is jumping. The double jump would allow you to choose when to employ a low, single jump or a high, double jump. This will allow you to “juke” your enemies in the Z axis as well as the X and Y by traditional strafing.
The double jump would incorporate well into the existing movement capabilities. Sprinting before jumping will allow you to travel further horizontally. Double jump would add to that. Sprint then jump then double jump then crouch for the furthest possible horizontal jump.
Jump then double jump then crouch for the highest vertical jump.
The reason why double jump is good for gameplay and the Jetpack is not is because the double jump gives you some vertical mobility without allowing you to negate map control by flying high above everyone, rendering their cover useless. It makes your movement in the vertical plane somewhat unpredictable. You have the choice to strafe left or right on the ground. Why not have the choice to jump low or high in the air.

