Kassadin will forever be permaban as long as they keep him an assassin (long read, ideas inside)

Kassadin's mobility means that he's generally going to control the pace of a fight and only pick fights when they're a good opportunity, by giving him burst, he will appear next to you and insta-kill you before disappearing a few seconds later, this combined with his extremely long-range pick potential from his RE combo means that he's not very fun to play against since you can't catch him, you can't run from him, and even if you had a chance from running from him, your health bar is null and void anyways.

With that being said, his kit doesn't need major changes; he has interesting abilities and they're not all completely toxic. While his ult will always be a bit toxic to deal with, it's probably better if Riot makes him pay for this power by making his damage require time and commitment to put out. This is why I think the following kit changes would be good for him (I'll justify it as I go along but this is just a summary of why he needs changes):

The actual change parts without justification have been bolded for those that don't care.

Passive - It should be the old passive. For those that don't remember, it still had the 15% magic damage reduction, but it gave AS based on damage mitigated by the passive at a rate of 1% attack speed for every one damage. While this sounds powerful, it wasn't really that strong since 1% AS isn't much, and with how squishy he generally was, a Kassadin would be near-dead by the time it gave him absurd amounts of power. This might need a bit of weakening since the kit is focusing on tankiness, but you can probably fix this by swapping the priorities of his passive with magic resistance, although that feels punishing.

Kassadin gains 1% attack speed for every 1 damage mitigated by his passive, 15% innate magic damage reduction (applies before MR)

Q - His past incarnations of his Q have always been rather boring since they're a ranged damage spell with some other effect on them. Whenever I talked to other Kass mains, they all said they want his silence back in some form since they like the idea of void being silence or something, but silence + blink + burst is bad so this Q will be rather awkward in execution and require Kassadin to have some forethought. It's also a nerf to his pre-six laning phase which honestly isn't too bad since he can bully quite a few mages and still farm well, but oh well, it's a casualty for what's hopefully a greater good.

Kassadin creates a barrier of null energy around him, shielding him for X amount (all damage, not just magic), when the shield breaks or expires, everything around Kassadin is silenced for X seconds.

It's completely a utility spell and you can't reactivate it so Kassadin has to decide whether he wants to go in immediately for a more reliable shield or go in later to get a faster silence. Since he controls the pace of many fights, this'll give him some more interesting options.

W - This needs little changing and I'd imagine it'd be the bread and butter of his kit. While I don't think he should just be an AP version of Irelia, the W damage should be buffed, and it should be changed to the old W where it gives him mana on every hit (reduced against minions) and activating it gives him a sizable amount of magic damage on hit. Perhaps %health damage if he's unable to build deal any damage at all. The current version of his W, however, is extremely broken since the mana restore could be extremely high (there's a specific late-game Kass build with around 3.5k mana that was literally always able to ult whenever his W was up.)

Every auto attack passively gives Kassadin a small amount of mana, greatly increase against champions. Kassadin can activate this ability to gain magic damage on every auto attack.

E - This ability would probably also see little changes, but the base damage should probably be increased a bit since his Q bullying has been removed.

**No change (maybe base damage increase) ** R - I think with the nerfs to his burst, the old style for the ult could work if the ratios were changed. For those of you who don't now, the old R stacked up to 10 times with 100 base mana cost, increasing by 100 for each stack and gaining base damage/ap scaling. The CD, however, was two seconds longer at each rank, which would come back as well. The current ult wouldn't really be fair to him with the W going back to what it was before since that's the only ability that was definitely buffed with the other rework, and his mana regeneration would not be reliable enough unless the W was massively buffed in every aspect with the changes I propose.

Now stacks up to 10 times, scaling starts extremely low at like .2 and gains X base damage, .1 scaling, and 100 mana cost per stack. Stacks last for 12 seconds. CD changed to 9/7/5 from 7/5/3, mana scaling was removed, a percentage of the cost is refunded for hitting a champion (this last part might not stay depending on what they do to his W, assuming they even do this, but it was there before the rework with the worse mana from W)