Or just increase the total amount of recasts after he gets a kill or assist.
Say he ults, gets a kill , ults gets a kill, ults gets a kill ults ults gets a kill, ults
The catch is he has to ult before he gets a kill / assist in order to earn the recasts and he is forced to use all of those recasts within 10 / 12.5 /15 / 17.5 / 20 etc. seconds of his initial ult or else it goes on cooldown and he wont be able to use the remaining charges....
with 10 seconds being if he didn't get any kills, 12.5 if he got 1 after ulting. etc
It rewards a good khazix player...
It makes it easier for a khazix to do well in a group fight if the enemy plays poorly and lets him get away with picks...
Not that khazix should be good in groupfights, its just that he should be good at punishing mistakes...
It makes anyone who is playing against him more reliant on cc, so therefore tanks will be more important against him (since his R usually wont be enough to stop him from being cc`d by a tank)
And its not contributing to damage creep unless the tank does not do their job in which case he will get more damage but over a longer period of time.