Too late for the contest, but a nice idea popped into my mind too.

Name: The Deflection Shield (or the Reflection Shield, the Mirror Shield, the Force Field Shield - pick your favourite).
Visuals - Body: Generally, the shield is transparent/invisible. A very faint version of the green thing in this picture is visible at the side towards which the player is moving. The colour could be neutral/white, maybe this is best achievable with specially-shaped lighting effects. The effect should be more pronounced at higher speeds, but it should never be brighter than about 30% of being completely saturated.
When an 'enemy' bullet (i.e. not from you or your team) hits the outer border of the shield, that part of the shield lights up brightly with the same shape like the thing in the picture. The brightness of that should be 100%, which I envision to be comparable to the explosion of Pepper Spray bullets.
When a 'friendly' bullet (i.e. shot by you or a team mate) hits the outer border, or when any bullet hits the outer border from the inside, a similar effect should be seen, but with only 50% of the brightness.
So basically, the shield is only visible while moving and whenever it is struck by bullets.
Visuals - Projectiles: The shield does not have its own kind of projectiles. You just shoot Blaster bullets if you select the Blaster. Any bullets that hit/pass through the shield's outer border get brightened up a bit (extra lighting). This is to visualise the fact that the shield 'did something' (see below) with it.
Visuals - Particles: The shield does not produce particles.
Visuals - Monitor: The monitor could show a very faint/incomplete circle with one bright side that spins around (much like what the shield would look like if the player would move in circles).
Mechanics - Projectiles: Instead of producing its own kind of bullets, the shield affects projectiles that touch it.
*All 'enemy' bullets that hit the shield's outer border are reflected. This reflection is in general like a mirror. That is, reflection around the normal of the surface, basically changing the sign of the angle of incidence. The speed of the player carrying the shield is added to that of the bullet after the reflection. Note that this may again change the direction of the bullet: if the bullet would be deflected exactly upwards if the shield was not moving, then if the shield was moving right, the bullet will move up and to the right. Most natural, as this gives the effect of 'pushing' the bullets away.
*All 'friendly' bullets that hit the shield's outer border, as well as all bullets that pass the shield's outer border from the inside out will pass through (hence the reduced lighting effect in this case), but they are deflected by changing their direction towards the surface normal by 50%. The bullet's speed is not changed in this case.
*Pepper Spray bullets do not follow the above. Since they are so small, the shield does not react to them at all - they are completely ignored. Enemy Pepper Spray bullets will do their normal damage if they hit the player character.
Sounds - Looped: Maybe a constant, subtle, low-pitched humming would be nice. But silence is fine too.
Sounds - Projectiles: When projectiles interact with the shield, a more pronounced low-pitched hum is good. In general, the more visible the shield is, the more it should be audible as well.