Katarina is still the most OP Snowballing Champion in the game
She has the highest AP ratio on her ultimate, she gets an automatic cool down reduction on kills/assists, and she gets damage reduction. No AP Caster/Assassin/Mage gets these kind of free stats. Ever since you changed her kit, she has become more OP. She was more balanced during seasons 1/2 than any other champion. Why? Because she had a high skill cap, understanding how her kit worked and knowing when to use her passive abilities, is what made her a strong champion. Every other assassin in this game, that snowballed this hard has gotten nerfed. Akali, Diana, Zed, Talon, Evelynn, Kassadin, Ahri, Kha'zix, Rengar and LeBlanc. They were all very strong for 2-3 patches at time before being fixed. Katarina stands as the most OP champion in this game because of her free 15 second CD. Either change it to a % of the CD per ability, nerf her ultimate AP ratio or completely remove her passive for a new one. Currently when Katarina hits late game, nobody can outrun her ultimate. Sure you can CC her, silence her, lock her down during her ultimate...but in a team fight, after 2 assist + 5 seconds passing by...she can use her ultimate again and by that time you're either too low from before, your cc ability is on cd or you are already dead.