It's easy to think of historic cultures as discrete, isolated from each other as much by indexing boundaries as by physical geography; but in fact, at any given time, contacts between ancient civilisations could be pretty sophisticated. Trade goods would pass from hand to hand along the Silk Road or between ports in the Indian Ocean, and while most individuals would certainly stay within fairly narrow bounds from one generation to the next, those few who travelled at all often ranged quite widely over a lifetime. Think of the Crusaders, or the Seljuk Turks. One of the T'ang Dynasty Emperors of China had a general who'd originally come from Iran as a Nestorian Christian missionary – that was in the Eighth Century.
So: it's 12th Century Asia, and our protagonist is a wanderer, a mishmash of cultural influences – a product of two cultures, each with its own honour code and unique style of lethality; fully accepted by neither, he has carved out his own path and lives or dies by his fighting skills. He is… the NINJASSASSIN™!
I haven't yet decided whether he's a Japanese somehow co-opted by the Hashishin, an Ismaili raised by a Ninja clan for some reason, or a second-generation hybrid with a parent from either end of the continent: clearly, the backstory needs a bit of work to make any damn sense at all. But the name is awesome (in the modern, internet fanboy sense), the setting is rich and expansive, and the premise is ideal for a series of self-contained stories in the old pulp style as he travels from place to place. I'd probably need to team up with a writer who knows Asian history better than I do, but there's matter there to work with. Or am I just crazy?