Just imagine some good things about pirates. As we know, this kind is usually trying to steal gold and valuable things, so maybe, they need like a rogue/marskman style.
Skills Ideas:
"A Pirate Life For Me": As becoming a pirate, you learn to use Swords, so giving them like a 3% or 2% (now, it depends on devs) of crit with Swords, would be nice for them.
"Jackpot": Using a Blunderbuss, or even a gun powder, they use these weapons to do damage to the enemy. So, depends on what the Pirate uses, can be more or less DPS, but this ability could be used to do a insta crit; maybe 2-3 secs of charge its nice.
"More canons": Planting a Canon in the middle of the fight, like a turret that adds buffs to the allies, and debuff to the enemy. Optional, the turret can shoot a % of the stats of the Pirate
"Hide the gold": Adding an invisibility cloak, it only have 10 secs of the effect, but adds 30% speed player. That would be nice, thinking that pirates are like "thieves".