Riot have you ever considered making Kat's ult damage ramp up?
TL;DR make kat have some counterplay through how people on the other team work together.
Basically the idea is that instead of 30 daggers sent out each doing 35/55/75 + .25AP hittign the same target up to 10 times it starts at 25/40/55 + .20 AP
But each dagger that hits a target will apply a stack to that target making them take 10% more damage from the next dagger meaning that based on her first rank of ult 0 stack = 25 + .2AP 1 stack= 27.5 + .22AP 2 stack = 30 + .24 AP ... 9 stack = 47.5 + .38 AP Total damage is 362.5 + 2.9 AP to a single target. compared to 350 + 2.5 AP
But there is more to this then just slightly higher damage in a full ult. IF kat ults into 5 enemies then the max damage she will do to any of them is 187.5 + 1.5AP from 6 daggers.
This means that Kat players will have to be smart about going in on large groups of enemies since her damage will be effectively reduced by it.
Why do I suggest this? Kat is a snowball champ that when ahead is hard to do much of anything against. This is not going to make her unplayable but it will provide for some play patterns to make her less effective which should be the goal of all changes adding counterplay.
She will still be able to kill that Ashe who is off on their own over extended no problem.