I think that poison would be a great ability. I think medics should have 2 offensive abilities.
Giving the Medic poison (or any type of "unique" offensive skill) is a horrible idea. You will wind up with players picking Medic just for those abilities, and not actually playing the role of a Medic - like all the people who pick Medic in BC2 just for the M60.
The Medic is a support role, and thus, should get supportive skills. You wanna kill somebody, use your gun, that's what it's for.
As for Medic abilities in the game - Here are some things that I thought of:
Adrenaline Boost - Allies that you revive will gain a boost of speed for 3 seconds.
Epidemic - Allies that you revive gain a small health regen ability that will affect any nearby teammates for 5 seconds.
Second Wind - You have a self-revive skill with a long cooldown rate.
Overheal - Your medkits heal for full health plus 1 pip, but cost more to use.
Transfusion - You can convert ability points into health, and vice-versa.