[Suicide Junkie]

For REAL pirates, I believe some good traits would be:

-50% to all production
This means that ships will be supported more by capturing and scrapping enemy ships, and less from planets.

-50% to research
This means get ship capture, and steal the rest.

-9% to Reproduction
Pirates don't get along too well when there's billions of 'em, and we don't want them colonizing. Pirates should be capturing the planet, and then selling off the infrastructure for money, and then abandoning it before the empire's fleets come.

+20% off maintenance.
They know exactly what needs fixing and what dosen't, and can live with ships that are held together by spit and alien hides.

+15/+20 Aggression and Defence
Losing even one ship can be a major blow to small pirate organizations, so it better not happen often.

Definitely have Advanced propulsion, and maybe Temporal for the shield depletion effect.

Mods required section:
Free resource storage with each ship/base.
OR
Resource storage components for ships/bases.
This way, you can keep your treasure without need for a planet.

A racial trait, where the pirates can immediately research Storm Generators, so they can hide their ships & bases.
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Basically my idea of an Ideal Pirate race, would be one that lives by stealing everything from other empires, and hides bases out in nebulas & storms for resupply & retrofitting.

The reace would probably not colonize, and would have its homeworld hidden in a storm or nebula.

Naturally, only a human player would be able to pull this off.

Maybe a multiplayer game, where you play against AIs to see who can build the largest criminal organization.