Have you tried using Kha'Zix top/mid?
His early game is somewhat weak, but once he finishes
he becomes very strong. At SoloQ battles it is very easy to pick people off.
Also, although his early game is weak if your opponent plays correctly, you can poke people with passive + Q. Many people don't expect Kha'Zix to do so much damage. Kha'Zix's lack of waveclear earlygame can be an advantage if your opponent pushes, as it allows Kha'Zix to get them isolated. This can potentially allow Kha'Zix to get early kills.
You may ask if there is any reason to pick Kha over other assassins, for example, Zed and Talon. Kha'Zix advantage over Zed is a better late game, as Zed's damage can be completely negated by QSS and Zhonya. On other hand, Kha'Zix's damage can only be countered by proper positioning. While he is reliant on getting targets isolated, it is not like other assassins don't rely on picking people off. Over Talon, Kha'Zix has an advantage of having better early game, better poke, and the fact that he isn't forced to build full damage to be useful (I build some tanky items on Kha'Zix, namely
)