I see a lot of people keep asking if the devs had any plans for the game related to the Kestrel. So I'll write this post what I found so far.
Splinter Cell Blacklist was a financial failure, so Ubisoft decided to stop post-release support for the game. This means no patches anymore or possible DLCs. But the devs originally planned the DLC during the development and I have proof on it.
This thread contains small COOP story spoilers!
As we know Sam and Briggs after completing the 4th COOP story mission rescued VORON's agent named Kestrel who was in a coma. This is where his story ends, leaving only hints of his future fate.
The first I managed to find in the game files (in the conversation.xml which contains all the game's subtitles) are a few lines about recovering the VORON's agent from a coma on the Paladin. Grim says that the Kestrel's up and awake in the infirmary and ready to talk. Sam goes to the hospital area and asks how Kestrel is feeling, but Mikhail is suspicious of the Americans. Considering what happened to him earlier. After some time Kestrel becomes more accommodating and tells Sam everything he knows and asks for help. This is the plot for the planned DLC.
These lines were even voiced by actor Alex Ivanovici:
The second what I found was his playable model. I'm talking about the playable model (not the NPC), because there are three of them (Sam, Briggs and Kestrel). In the game there is a difference between game models only in the head, i.e. the model of the suit is the same for everyone. After a little tweaking in the PEC.ini file I changed Sam's texture to Kestrel to see how he looks like.
There are also several lines in the loc-int.umd file confirming that Kestrel is a teammate in COOP mode.
And the last confirmation proving that the devs originally planned post-release support for the game as DLC (as downloadable content like The Insurgency Pack for Splinter Cell Conviction, not as unlockable disk content). If you follow the link to slide 4 and zooming on 400% of the official developer presentation for GDC, you can see the Kestrel head DLC in COOP section. This slide also confirm 75-sp1 pistol as DLC and 8 ADV maps were planned:
https://www.gdcvault.com/play/1020212/Super-Spy-Crash-Course-Teaching
Kestrel's 4 missions had a new type Assassination (2 missions of the Hunter and 2 missions of the Ghost) confirmed by the SC6EconomyMatrix.xlsm file in the COOP Missions tab (which can be found on game directory if you install the game version 1.00 without patches), it contains information about the game's economy, weapons, missions, etc and the loc-int.umd file which also contains some lines.