Im not really sure that my suggestions would make katarina better or more compatible with other players but i would like to put a few things into consideration.
*Kats shunpo could target turrets.
*Kat gains additional movement the more enemy champions hit by sinister steel.

And finally iam not sure that this is the right place to mention this but i do know that it would make kat amazing.

*For the love of god change that stupid slay belle recall (chopping a candy cane) its not cool.
Thank u for ur time

I don't play her much anymore because she's a very polar character (you're either going to do very well because of a lack of cc, or really bad because of an abundance of cc), but I do think she's balanced. Shunpo is a wonderful escape skill. You can carry wards to give yourself distance like Lee Sin does. You can also do fairly well without your ult, but she requires you be close to your targets in any case, so cc is your mortal enemy in any case. And, unless you're fighting a very beefy bruiser (think Garen or Darius level), you're most likely going to win a 1v1 situation. She has her sweet spot.

Reminds me of the Frost Mage from WoW. Borderline OP when their is a lack of dispels [CC] but vastly diminishes in strength if there is some to the point of being Borderline UP.

She doesn't need any buffs but I think you hit on to a major problem in LoL. Champs that aren't that good at damaging turrets are not really compensated for it.

Definitely NO.

Kat is fine the way she is, realistically speaking. If it were my way, I'd nerf the piss out of her. As HereticSage said, she's very polar - either really dominant or very bad - though I see the OP side of her way too much. I'd like her ult to have a longer CD, and her to not do so much burst damage in general. She's like a melee LeBlanc.

This buff would be too much. She would just W and get away in situations she should never survive

Nope. She's borderline OP right now already. Being able to escape to turrets and getting bonus speed from abilities makes her even more OP.

As Mace said, I'd nerf her if I had things my way. She's like a LeBlanc with a slightly weaker early game and a much stronger late game.

OP needs to L2Play.

Leave Katarina alone please, she is perfect.
I have grown so tired of "OP" and "UP" complaints.
Her kit is one of the best Riot has put together, leave her alone.