Kassadin will never be reworked properly until his ultimate is changed
I can't stress this enough, reworks don't need to keep identities if it's only hurting the champion. This is the exact reason why the Sion rework was a major success; it removed the anti-fun and toxic parts about him and revived him into a champion that can be number tweaked if he is either too strong or too weak. There is practically a 0% that Kass can be a viable and strong pick while not being oppressive with his current ultimate.
Pre-rework kass was basically the ideal midlaner, someone who could roam and snowball his lane effectively; better than any other midlaner. All his rework did was basically shift power and lower a lot of his damage, it didn't solve the problem of him constantly being nerfed because of his ultimate. Kassadin has been nerfed so many times to the point where he does no damage and he's just outright useless.
When champions like akali,shen,poppy etc get reworked they will need their ultimates to be severely changed for them to actually become balanced champions, there is a huge reason why they can't be viable and it's the toxic parts of their kits that holds them back.