Question about Kai'Sa

Why doesn't she have an evolution for her R? Something like when she reaches X amount of mvspd/crit, the shield value is increased by X percent based off of mvspd/crit? or instead of a dash it becomes a blink or she does X amount of damage upon landing? If you're worried about her potentially having access to 4 evolutions then why not just limit it at 3, not to mention, no one evolves w anyway. Kai'Sa is in a pretty good spot number wise (although i wouldnt mind a slight range buff), but i think her kit needs some slight changes and i feel that this could be one way to go about it. The other way would be to lower the attack speed requirement on evolving e so that you dont need one whole zeal item, berserker's greaves, and a dagger to evolve it. There are games where it is sub-optimal to build berserkers (full-ad or against ad-assassins) as well complete another that requires dagger.