First Idea: An item that allows friendly explosions to launch you. I can imagine insanely fun teamwork with this. Like launching a hailstorm of heavies. It would probably replace your melee weapon similar to the mantreads.
Second Idea: A spy watch that teleports you a short distance forward. It would have a short recharge rate, no invisibility, and possibly multiple uses like the vaccinator. It would be great for escapes, or teleporting behind enemies. Think the rogue's shadowstep in World of Warcraft.