Having never been pirate i cannot say for sure but i understand that there are far less perks for flying the Jolly Roger than a given nation flag.
I propose a new Pirate only Perk, there are 2 ideas below but i'd suggest one or the other, some will likely want both
One
It would have sometimes been hard to tell one pirate from another out in the seas, you would only know and fear the flag
Fear The Flag
Once per day a Pirate can make a single undercover attack on an enemy fleet. The pirates fleet still gains danger but the person that was attacked will only know it was by a pirate and not the player.
This should be maxed at 1 per day and reset during normal update.
Could add a new button alongside the current 2 options on the Plunder page next to each fleet
Two
Pirate Spent a long time at sea looking for targets and sailing in circles. They would have found hiding places and coves no-one else knew existed.
Smugglers Cove
Once per day a Pirate can remove X danger from a single fleet.
The danger removal would not be able to be split across multiple fleets, if the fleet you are removing danger from has less than the total you can remove the rest is lost.
I would propose somewhere between 6-12 danger be what can be removed using this perk. 6 Danger gives a single attack and 12 isn't quite a hideout card.
This should be maxed at once per day and reset during normal update.
Either that OR a pirate can perform a single attack with no danger increase once per day.
This could also be achieved with the addition of a new button on the plunder screen.
The 2 options above would either give a pirate the chance to perform a single attack anonymously or run and hide after performing an attack, something pirates probably did and relied on to survive.