There are two subcategories of Slayers: Assassins and Skirmishers.
We all know what assassins are. They get in, deal extreme burst damage, and get out.
Skirmishers are the other side of the coin. They don't have burst, but they deal a ton of sustained damage. They may not be able to kill you instantly, but they can chase you to the ends of the Earth, dealing heavy damage the whole way and surviving in the fray with their defensive abilities.
This is Akali's inherent flaw: she gets the best of both worlds. She has powerful defensive cooldowns for surviving in the middle of the enemy team like a skirmisher, but heavy, front-loaded damage like an assassin. Her time to kill is a bit longer than other assassins, but not significant enough to call her a true skirmisher.
I feel like the skirmisher playstyle would suit Akali so much better. Lower her damage, but compensate with better energy restoration on her passive. Let her have her sustain back, even if it's only against champions (so she can't just live in the lane forever). Make it so that she can't delete people, but when she picks a target, she will stick to them and keep coming at them until they are dead.