My idea:

The Runner's Radio
Level X Communications Device (Secondary)

Use to reveal nearby enemies to you and your team for 10 seconds. (gains a team-coloured glow like in respawn, affects all enemies in a sentry-like radius, marked enemies that are killed by your team award you assist points)
-) Charge meter by doing damage (deal 300 damage to charge, meter can be charged while the effect is in use, prolonging the effects)
+/-) Meter charges faster on kills and assists (gain 80 bonus damage on kill and 50 on an assist, encourages team support and rewards consistant kills)
-) Meter starts empty and cannot be refilled by resupply (like the gas passer)

The ability to see enemies through walls is a bit too overpowered, so I've tried to dilute that with a gas passer style charge up. The bonus meter charge for assists and kills works when the radio has been used as well, meaning that killed enemies reward more time for the radio effect.
I feel this may be overpowered in competitive, since it is easier to position yourself to get picks. The major downside of this weapon is that you have no secondary weapon, meaning that aim with your primary is essential to get that meter full and unleash a powerful supportive effect.

Let me know how I can improve this idea!