Well the thing is, is that you already are a combination of Ninja and Samurai. You have your signature blade(Samurai), You rely on stealth to give you an advantage over your enemies(Ninja), You have a code(Samurai, could also apply to the ninjas, but I'm not as familiar with them as I am with samurai), you kill the leaders and the important figures of your enemy(Ninja), you defend the innocent against the oppression of your enemy(Samurai, or at least that's how they'd like to be remembered), you have many different gadgets to get your job done and you don't get all tied up in honour in how you kill your enemy(Ninja). That's essentially what the Assassin's are, they are Ninja-Samurai in a sense. They take the strong moral code and philosophical aspects of the Samurai and mix it with the brutal effectiveness and clandestine nature of the Ninja. You've already got what your Ninja-rai, time for the pirates.