At the current moment i believe katarina needs some changes, specifically because she is really strong in low elo but almost useless in higher elo. People in low elo don't know how to counter katarina (saving your cc then using it when she ults), while people in higher elos know how to counter her (interupt her ult). and it counters her way too hard. I was thinking about it for the past couple days and i think i know what you can do to make her more balanced in lower and higher elos.
The first change needed is to her q, her q needs a buff on base damage, maybe buffing it later on and not in the early levels. And maybe even change the scaling to 65%.
Another change that could have use is a nerf to her w base damage, and remove the ad scaling from her w and r.
The last change is to her ultimate, I believe she is counter really hard by cc because she rely's way too hard on her ultimate, therefore buffing some of her basic abilities and nerfing her ultimate will allow for less need for her ultimate, and more need for her basic abilities (which means less countered by cc) buffing q could help her laning a lot too, as she has quite a bit of hard matchups in the laning phase. some other cool changes could be reduce cooldown on e, and buff movement speed buff on w.
Also with the suggested changes she would not be as much q-e-w-r and melt everyone until you're cc'ed, and more strategic planning on what you're going to do in teamfights.