I think I'd be great if pirate ships spawned based on say mass. Say the server admin has 5 different custom pirate ships that he marks as pirate usable, a 500 mass fighter, a 2,500 mass cruiser, a 10,000 mass battleship, 25,000 mass dreadnought, and 80,000 capital ship. They would spawn in frequency based on mass, so the fighters and cruisers would be common sights, the the battleships and dreadnoughts more uncommon sights, and the capital ships rarely ever seen.
Maybe have the percentage chance adjustable by server admins, so I could mark that fighter as 75% of spawned pirate ships, the cruiser as 10%, the battleship as 7%, dreadnought as 5%, and capital ships 3% of spawned pirate ships.
Maybe have the percentage chance adjustable by server admins, so I could mark that fighter as 75% of spawned pirate ships, the cruiser as 10%, the battleship as 7%, dreadnought as 5%, and capital ships 3% of spawned pirate ships.