My schedule's starting to get less busy, it seems, but we'll see in the next couple of days. Either way, I actually do have two servants in mind, with one not canon, and the other sort of partially canon.

The first is Hattori Hanzo, who's probably the most famous ninja in history. He was also a samurai, so he fits the bill for a combat-oriented assassin.

The second is Bellerophon, who would be a Rider. He's the "sorta partially canon" servant I was referring to, as there's a Noble Phantasm named after him. I'd expect he'd use the same Noble Phantasm, with it possibly being stronger than when used by Medusa.