It strikes me that Pirates don't actually behave like pirates at all. All they really amount to is a Mercenary force that goes and attacks whoever has the biggest fee on their head. Essentially they work for whoever pays the most, which is Mercenary basic code!
My idea I think rename the pirates into mercenaries or change the behaviour so they act like real pirates might behave. Instead of going for whoever has the biggest bounty on their head, pirates might go do things of their own accord.
- Have a small pirate force free roaming and attacking/stealing resources
- Pirates raid players based on how well players are doing. For example in a 4 player game if the pirates might raid the top two players (the ones with the most credits or resources/assets)
- Pirates demand protection money sometimes or they might attack.
- Pirates raid trade ships.
Something like that would make the pirates more "pirate"like instead of mercenary.