Seriously, just why? I don't get it. Why would you give an assassin completely free sustain for doing nothing but chucking an ability at minions? And it's not like it's a little healing. I've seen Akali's heal over half their health with a gunblade simply off one Q. It doesn't take any skill. I'm okay with her being slippery, with her ultimate, and everything else in her kit. Even the (kinda stupid) "true stealth". But why give her such an easy accessible heal? Old Akali healed, yes. But after her changes to balance her out, it was only on an auto attack. And it wasn't nearly as considerable as it is now. Laning against Akali, seeing as she has no mana, is basically 100-0 her, or just get pushed out of lane or dove eventually. You will NEVER out sustain her on any champion. It's just not possible. As far as I know, there's only two items that are anti-healing in the game. One is a late game armor pen item, one that you wouldn't really want to grab early, and the other one only works below 40% health (or something like that, idr the numbers), which Akali rarely gets to because the second you try to do any meaningful damage to her, she shrouds and runs away. Then she can Q back to full health.
Easy proposed change: Put the heal on her empowered auto attack. Focus her kit more on her all-in potential and aggressive, slippery play style. Instead of being able to Q the minions and heal a bunch for free, she has to actually get in range to hit you with a Q or E and then auto attack something with the empowered auto. It would increase her outplay potential, since she could suddenly get a heal after hitting her E and diving onto you. It'd feel more fair since the player could play around it, and could offer more counterplay to champions that can keep her back.