Cold-blooded wasn't a problem because of two reasons. Stopping power in the same tier and Heartbeat Sensor as a counter.

Ghost had counters, the Blackbird and motion sensors.

Assassin needs to have decent counters. My suggestions: Advanced UAV, Marksman and Heartbeat Sensor. Move HBS immunity to another perk, like Dead Silence.

Assassin already has two conuters- Recon perk and recon drone.

Neither is good enough.

To be able to expose an Assassin user with Recon you need to know where he is and use up your very limited supply of explosives/flashes. And the effect only lasts 10 seconds (5 seconds if he has Dead Silence too).

Recon Drone? So you earn one after 10 kills, again you need to go and find the Assassin user and manually tag him. The effect lasts until he dies and then he's hidden again. Not good enough.

There needs to be some passive counters. Marksman should show the name above head even if the enemy is using Assassin Pro. Advanced UAV should show up Assassin users aswell (it only takes one Stinger to take down, so it wouldn't be op at all) and Heartbeat Sensor should counter it so you could have a constant passive counter like you did in MW2. Another perk should in turn counter HB sensor.