For blue Kayn just take electrocute. He already moves faster than a Kenyan. Plus if you take ghost blade and mobility boots. You don’t want your opponent having the heads up (besides the indicator when you’re in a wall) that you’re coming. And he already does a ton of damage so predator would be overboard. Build lethality. You won’t need the crit. If they start building armor either get last whisper or black cleaver.
For red Kayn take conqueror. You basically never die. Get black cleaver after jungle item and then probably either GA or spirit visage depending what you are fighting. Then get deaths dance and you’ll be unkillable.
With kayn’s E You basically are bard and talon’s love child so use unique gank paths. Also you can get over some walls with q so save your e if you can just q over the wall. But anyway look for the lanes where you have allies with cc in them and gank from behind so you can get as many hits on them as possible. You wanna try going for the transformation ASAP and then you can farm and scale more. But remember take this with a grain of salt and be flexible.