Level 1
Preparation - Backstab: your next backstab have less CD (or no CD).
Preparation - Deadly poison: your poison makes healing on the target less effective (?%).
Preparation - Shadowstep: increase shadowstep range.
Level 4
Deadly poison (E): poison spreads to nearby enemies.
Critical wounds (D): while preparation is active your first basic attack on the target enemy hero deal +5% of it's max health as damage, 10% from behind.
Hide: you can hide in enemy structures, gaining a lot vision on the area.
Assassin's power: killing an enemy hero with backstab gives you permanent 3% ability power.
Level 7
Follow through: after using an ability your next basic attack deal bonus damage.
Mana drain: if you use shadowstep before backstab, your backstab drain mana from the enemy hero.
Vampiric stab: if you use deadly poison before backstab, your backstab steal health from the enemy.
Sprint: if you use shadow step in the last 2 seconds of preparation, you gain movment speed bonus.
Searing Attacks: increase your attack speed for a short time.
Level 13
Deadly stab: backstab deal half damage, but if shadowstep used before backstab deal 4x dmg from behind, without shadowstep it does double damage from behind.
Poison overload: you add more poison on your daggers in the last 2 seconds of deadly poison duration. Dealing a backstab to the target during this time stun the target for 1 second.
Safe step: backstab an enemy, than using shadowstep gives you a shield. You can use shadowstep an allied heroes and can store 5 stack of shield, which reduce the next incoming damage by 50%.
Level 16
Double step: using preparation and shadowstep before backstab, refresh the CD of shadowstep.
Weakening strike: use preparation and deadly poison, than your backstab cause the target to vulnerable (35%).
Fan of knives: throw knifes around you, slow enemies.
Level 20
From the shadows: using Vanish during preparation and your attacks wont break the invisibility.
Drop: using Disarm during preparation and the enemy's weapon land on the ground. After the disarm duration the hero have to pick up it's weapon. After a short time the realm lord give the weapon back to the hero.
Master assassin's blade: deadly poison stay on the target for 30 seconds.
Dodge: while preparation is on, you dodge 100% incoming damage, decaying to 50% at the end.
Heroes of the Storm Build Guides
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Other problems include complete lack of survivability for a melee hero until 13 and preferably 16 (otherwise unusable if you didn't plan to initiate with Vanish), no greed mechanics (nothing like Greymane's Inner Beast, Thrall's Resilience, Artanis' shields, Valla's Hatred, Seasoned Marksman) for an AA hero.
Furthermore, her abilities seem too simple and streamlined, as if there's only one way to play her, by using W-E-Q over and over again. You know what this reminds me of? League of Legends.
She scales very poorly, is liable to die for any attacks she makes, lacks a role to fill on a team (can't really harass that well on a 15s cd with no survivability), lacks skill (no skillshots, no greed).
Many of her talents need to be redesigned.
Preparation: Other heroes have talents like these, except those are always active. The way you've interpreted Preparation is too weak. Compare to Kael'thas' Verdant Spheres, Illidan's Friend or Foe talent, Xul's Mortal Wound talent (which is L20, but is always active, on a ranged AoE, and the ability can have its cd reduced).
Critical Wounds: Randomness. Instant no-no.
Hide: Is there a timer? Can you be removed? Seen? How do you even get close to enemy buildings, and how do you guarantee a safe escape?
Gathering Power: Only for mages. I don't see a Mageatul-like build here, too many talents needed for survival (1, 13, 16). Poison damage is not affected by AP.
Mana Drain: Removing mana is useless in most cases. Very few heroes spend much time low on mana AND need mana to escape or survive.
Searing Attacks: Blizzard has chosen to remove most generic talents, and I'm not sure how many heroes still have Searing Attacks. Don't introduce it into a new hero.
Safe Step: Inferior to Spell Shield and all Spell Shield-like talents. Does not activate when you need it. Only useful for Shadowstepping friendlies. Is Shadowstep on a low enough cooldown that you can ever afford to use it offensively again, tho?
Double Step: Just one of many combos that shut down creative use of basic abilities.
From the Shadows: What does break invisibility then? Can you even be revealed with AoE? Are you just permanantly Invulnerable while Vanished?!
Drop: Not all heroes use weapons, e.g. Brightwing, Diablo. Btw, can Kharazim still Dash to his allies? That's a damaging ability, but not when used on allies.
Master Assassin's Blade: Too long, too strong. Compare to Artanis' Purifier Beam.
Dodge: Requires you to save a 15s cd for survivability. Effect translates as "Protected", as with Medivh's W, but lasts too long to be balanced. Fortunately for you, it imbalances in the right direction, since she's too weak in all other ways.