Currently, assassins gain the feat Weapon Proficiency (Assassin), and can wield the following weapons without penalty: Kama, Katar, Sai. I propose Wakizashi to be added to the list.
Wakizashis are thematic to the other Eastern weapons on the list, and would also be an appropriate weapon to be used by an assassin, that is not covered by its current simple weapon proficiency (such as daggers and sickles).