Disclaimer: I'm a Katarina main. Take that as you will.
I agree with you thoroughly. Kat is too strong at the moment, and definitely needs looking at. If not only so she can stop being banned.
But, is it the champion that's too strong? Or is it the tools she's given?
Let's look at Kat's kit right now. High damage, high mobility. This means snowball. Snowbally characters generally don't have CC right? As far as assassins go, the strongest hard cc available is a 1.75 second stun from Ekko, which is incredibly hard to effectively use. So it goes to say that a general rule is assassins shouldn't have [much] CC.
tl;dr: Kat is a snowball champion with 0 CC/healing; intentionally, with extremely high damage potential.
Now, let's look at the current itemization and runes offered to her.
An average rune page for Katarina consists of Domination: Electrocute - Sudden Impact - Eyeball Collection - Ravenous Hunter. Precision - Triumph - Coup De Grace. Some people take Last Stand>Coup De Grace, but it's personal preference. Now tell me what you see?
Just 3 runes alone are based off snowballing, 2 of which heal for doing damage[essentially]. Almost all of them help her already high burst. Put these together, and you have a snowballing monster who heals wayyy too much simply for doing her job as an assassin.
Now to look at itemization. This will be short.

This item is a huge issue on Katarina. Akali too, but that's for another post.
Gunblade gives about 120 in stats to Katarina since she has hybrid scalings, heals for 15% of all her damage. (Giving her 30% total when she finishes ravenous hunter stacks, on top of 12% missing hp on takedowns from Triumph) and a 60% slow for 2 seconds.
This is huge. Katarina was designed to have neither healing, nor cc in her Kit. This item gives her both, plus 120 stats worth of damage.
Can you see the issue with this?
tl;dr: I don't want to see Katarina get all of her abilities dumpster'd, only to have them nerf her core runes/gunblade later down the road and just Olaf her.
Sorry for the wall of text, I have a lot to say about Katarina's balance state.
Edit: Keep in mind they're most likely changing precision to give AS in combination with other pages, which should actually hurt Katarina. She won't be able to take Coup De Grace/Last stand and Triumph without losing early damage.