I am Kayle main, had 800 ranked games as her in season 4 and 300 in season 5, reaching fairly good rank in both of these, with this old kit then I was inactive for 2 years.
Now when I came back, everything in game has so much damage that her damage is nothing compared to it (she needs to get fed, or at least 4 items to work). Her utilities come at cost of damage and her damage comes at cost of utilties which results into having both aspect at 50% potential. Her kit is so incompact and only combo is Q + E.
So she lacks damage, mobility (unless you play full AP), dodge, has no early potential and falls behind easily.
In my eyes she should be something like melee/ranged hybrid with hybrid rations.
What needs change in my opinion? Passive - it is so passive so boring, in my opinion passive would be much cooler if it was something like: under these and these conditions lits target with holy flame, making them vulnerable and taking small portion of damage over time
Q is fine I think, It could have been something offering a bit more punishment to greedy enemies, like part of the combo or something.
W... why should a punisher of guilty be a healer? No reason for it theme - wise, this is place for some cool self-mobility tool.
E - this makes Kayle unique, it should work similar as it does now, but look more cooler and for example become more skill rewarding, cuts could have been something like Graves's auto attacks, but slicing through everything and counting as melee and not counting as a projectile. Positioning and direction of attacking would matter, while target would be in the center of these fiery waves coming from Kayle's blade. The waves would be really quick so one should not be able to dodge it by moving. If used with titanic hydra, the hydra aoe would start from Kayle's blade too - making bruiser build viable. But it would still be dodgeable by blinks and so. TL;DR: Much stronger itself, dodgeable, missable, not requiring (and not working) with hurricane to be effective, but still would be great with attack speed.
R - her current ult is iconic, unique and sort of everybody got used to it. Here I am out of ideas. It could have been changed into some powerful boost like: Lits Kayle's blade even further and everything she did had much more purifying flames in it. Her E could have been only something like Fiery blade and R would be the Righteous Fury.
I have no idea how these would perform in game though, it is just my fantasy and only thing I know is that I want Kayle to lit her enemies with purifying flames and punisment.