Alright, let's get down to business; I've just had a flash of inspiration for a potentially dangerous Operative-exclusive ability to poison general Health-Kits (if there are any in-game).
Frankly, I have thought this through, and I can justify why it can be considered balanced yet effective.

To start with, general Health-Kits (again, if they exist in-game) will be found scattered around the map and will probably dispense a small amount of health (e.g. 10 hp) in order to prevent Medic redundancy. However, they will be sufficient to keep an offensive player barely operational in the case that the Medic is either incapacitated or too distant (which can happen if the Medic is incompetent, you're a bad player or an Operative behind enemy lines).

With his 'poison' ability, the Operative will be able to pick up a Health-Kit and ponder the choice to either use the Kit on himself or spike it to damage an enemy for as many hit-points that the Kit can heal. It will hurt anyone that uses it, regardless of allegiance; however, a poisoned health-kit planted by a friendly Operative will be apparent as such to his team by a visual device (a skull with or without crossbones should do the trick).
The catch is, however, hostile poisoned Kits cannot be distinguished from neutral ones, and will be impervious to even the Operative's preternatural visual detection. Also, they cannot be neutralized by any means.

To justify all this conjecture with its merits, this feature could prove to be the last word in abolishing gun-and-run play as it is excellent 'wolf poison'. Moreover, it will result in increasing the invaluableness of any Team's medic to greater heights. It will also fit in with both the game objectives and lore surrounding the Operative as a pre-eminent saboteur and assassin. Furthermore, adrenaline thrills gained by perilously gambling on your health as an Operative or a team-mate, both in the (hopefully) unfortunate absence of a Medic, may pay the ability's official admission sufficiently well.
As for the ability's balancing features, poisoned Health-Kits will probably damage substantially yet moderately if used well (i.e. it won't kill them, but they'll feel the pinch), and the act of poisoning will deny the Operative, and by extension his entire team, from taking advantage of it either as it is in essence a 'scorched earth' tactic. Also, the number of times it can be used should be limited by either a cool-down time or even 'poison' ammunition if this new mechanic can warrant further complex development. An ideal usage number could be three, limited in the aforementioned manner, to guarantee balance yet make it a useful ability in efficient hands.

Alternately, one could consider the similar option of poisoning Medic Med-Kit pickups, as their Med-Kit ability is probably fuelled by specific ammunition. However, significant drawbacks to this include difficulty of regulation and disposal (as it is harder to keep track of and discard tainted ammunition), overtly damaging effects as a subverted Med-Kit could become a 100 hp OHK. Further concerns are that it will be monstrously unjust for any Medic in possession of tainted wares as his expertise is reduced to dangerous quackery. However, given that the Operative is, after all, a gifted saboteur, it would not be above him to try and break the win-win relationships present in good teamwork to create disunity...

On a final note, the inspirations for my theory were certain elements of gameplay in the games Team Fortress 2 and Bioshock; I'm sure you'll be able to guess the specific details.

None of the mechanics you have listed apply to the game really. There are no med-packs in the game at all. The only healing done is directly from a medic. Skills in the game all have an energy cost and energy slowly regenerates on its own. (think of it as MP or SP from rpg games)

So there is no time where a med-pack is on the ground, or even in someones possession.

Also, in reference to your poll, it IS too late to make a difference, the game is in closed beta now, and Splash Damage is not adding anything new to the game at this point, and are just tweaking, balancing, and polishing the game. Not to mention the fact that the devs don't generally visit this forum, and even if you could make a difference with such a poll, you would have to go to the Splash Damage Brink forums.

it IS too late, and you can't make a difference to the game through this forum anyway.
doesn't really fit Brink though, perhaps if there were more players and bigger maps, but i'd take that idea and spread it, there's bound to be a developer who thinks it might fit his/her game

i would prefer a chemist with a injektor, defense buffs and poison which kills slowly

I think the only possible direction that this could go in for Brink would be an offensive specialized medic with poison and debuffs, but as Brink is in beta, it would have to be done by the modding community.

Also, I'm not sure how it would be implemented, melee would be difficult to do, so possibly a dart gun or poison gas grenades.

There is supposed to be a medic bioweapon. It was mentioned in an interview. OP you really should read the mechanics of the game.

Only poison that I can think of fitting brinks style is a poison needle for Ops, requiring them to get really close. Poison needle would do little damage over time, slightly blur the screen and should be easy to heal.
However, I don't like the idea of poison at all.

its a cool idea, but if all these operative ideas that have been mentioned were implemented then the operative is going to have 2324123421 abilities

If he is close enough you use a needle he is close enough to backstab.

Could make it a skill to boost backstabs that gives a debuff after someone is revived or respawned, possibly either. Or maybe make it a skill that prevents a revive, possibly with a cooldown to balance.

Or, make it a melee skill for medics instead, though i should hope that the medics on my team are never within melee range lol

nah, doesn't seem like a good idea i think. i've been watching info on the Operative as much as i can, as its the class i'm most excited to play, and they've already got a whole kitchen sink load of abilities and uniqueness. maybe the medic could (or already has) something like this, but the operative's more of a spy i feel; sabotaging things on more of a team-wide scale with impersonating and such than little nit-picking.