Please remember that abilities come in 5 flavors: Engineer, Medic, Operative, Soldier, and General (anyone can use it).
Now the two that come to my mind are the operatives backstab and the medic's self revive and possibly the Lazarus grenade;
Backstab: Please note this has not been confirmed as a OHK, however if it is I can see a group of people who would think it is overpowered. I for one want it to be OHK.
Self Revive: I understand the Medic's need to keep healing the troops but this is pretty powerful. I think it will have a long cooldown.
Laz grenade: Similar problem as above, best way to balance is decent blast radius and long cooldown. It would be bogus to cut down the entire enemy team and then one grenade undoes all your work.
Feel free to add more and not I am not sure if these are even overpowered since we don't have the game. Also please don't flame each other and I am sure (hopefully) that we can have a civil discussion about an interesting topic.