Focus more on the Rifting aspect of Kass and drop the "anti Magic damage guy"

Because I honestly don't think its possible to balance someone that is both of those things nor do I think Riot really wants someone who is both.

LB and Nid are very mobile. They don't automatically defeat magic damage dealers while also being able to delete them by building heavily into defense. They don't have to have incredibly weak laning phases because of their mobility + tankiness + inherent defense against an entire type of damage.

Kass should be rifting in and out of fights like fucking crazy, make it even lower CD, lower mana cost. More stacks. Just make it crazy. Drop the shield on Q, bring back the Silence. Give him a passive that gives him Attack Speed or some other stat whenever someone uses mana or hits him with Magic Damage instead of giving him inherent Magic Damage reduction.

**You can still make him good against Mages without making him absolutely immune to them. If you continue to make him just straight up immune to the majority of their damage he can never be balanced or viable without heavy nerfs everytime he shows up. **