Cleave really wouldn't fall under a Ninja skill, more of a Samurai thing heh. Now Strangle, Garrote or Impale would be the kind of skills you'd find with a Ninja.
Other skills that could be applied to the class
Vanish - Disengage from combat and hide (IE, the use of a smoke bomb to perform)
Crossbow - Already in game.
Lightning Reflexes - Chance to attack on a successful dodge
And various others. Also for the Apply Poison, it could be built from it's own tree as spells/skills/songs are. Each status change in level could unlock a new poison that the Ninja/Rogue can use, however where as a Rogue can only imbue his sword/dagger with a poison, the Ninja can imbue his bolts for his crossbow.
Other paths you can take to inventing the class could be there own spell set that some Ninja's posses in other types of stories. Breathing Fire, Igniting their weapon with fire, attacking ones soul directly, Slipping into the shadows, and what not. Alot of ways you can go.
And lastly, you'll probably never see the class in game... ever. 7 years to create bards I'd say 14 for Ninja's considering all the additions I've seen in the last 3-4 years. So yeah, much like my Minotaur Monks, enjoy the pipe dream
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(Or play your Hellion as a Ninja, just swap your plate for leather)