Pirates should get stronger over time, have capital ships, maybe even one of their own. Ships of any factions should be spawnable as pirate ships.
I think pirates should be able to capture player ships - a channeled, interruptible skill.
Say that, besides having the ability of spawning any capital ship, it has also one of its own, dedicated to capturing ships. Smaller pirate ships will be the support for this ship. Sometimes they would ask for a ransom, to give back the ship. Captured ships which they would never normally spawn would always trigger the ransom (such as diplomatic envoy). The ransom should be less than 100% the ship cost.
A standard pirate raid would focus on getting money, ships and running away.
Pirate capship skills:
Overdrive - speeds up all pirate ships in the gravity well. This has two uses: fast approach and running away.
Takeover - skill to rob played ships, similar to the Advent's skill, but channelled, interruptible.
Jynx - similar to armistice, but with no money income. They jynx all weapons to be able to run away. Including their own.
Kaboom - Ultimate - passive. When the capital ship explodes, it explodes with quite an ooomph, damaging everything in an area. Similar to TEC starbase skill, but weaker.
Pirates can also do two other types of raid, but only if paid to do so: Destruction raids and terror raids.
They naturally cost much more than a common pirate raid, but also have a greater effect: With destruction raids you must give twice the money to beat a comon pirate raid. their focus would be destroying planetary structures and capturing/destroying ships. The terror raid is all about destroying cultural structures and bombing the planet to terrify the population, reducing especially the current goverment culture spread (if a government can't hold off measly pirates, how can it protect us?). It costs thrice the value of a common raid.
instead of just throwing money on their table and never get it back, the player should be able to set up a target planet for the pirates (including allies), raid type (more on this later) and change at will, anytime, putting or removing money from the table.
Players should be in the dark of which player is the target and who has put the biggest bounty. Some info may be garnered only with intelligence research, like knowing if you are the likely target before the attack arrives at your gravwell, and knowing if your bounty is the biggest on the table.
When the pirates gear up to action, There will be the usual waiting time. When they choose their target, they take up a portion of what everyone has placed on the table - the fee for listening to the raid offer - and all of the money put by the winner.
The amount the pirates take when losing a bid can be reduced with diplomatic technology.