This is a tentative rework for Kassadin, the Void Walker.
Overall, I think there's a lot that could be done to improve Kass right from the start: despite being an assassin, he has several effects that make him tankier against magic damage, which isn't the best way to handle an anti-mage kit, and several of his abilities are improperly gated (Force Pulse's spell stacking mechanism clashes with its innate cooldown, and his ult's restrictions are reduced by stacking mana, which isn't an adequate gate to his mobility). Kassadin definitely deserves to be both a premier anti-mage assassin in the game and the most mobile champion in League, and I think he can afford to fulfill this fantasy in a way that is fun for both him and his opponent, provided his kit gives him the right strengths and weaknesses.
Innate: Enemy champions generate Spell Traces every time they cast an ability, stacking up to 5 times.
Kassadin's strikes each consume a Spell Trace and deal 47 - 200 (based on level)(+ 100% AP) magic damage to his target.
Striking an enemy champion locks out all other enemies' Spell Traces until he leaves combat with his target.
I often get criticized for moving all of a champion's damage to their innate, which is also the case here, but I think there's a good reason for doing so on some kits: Kassadin, for example, is an assassin with a large amount of AoE spells. Taking out the latter part would harm him, since he can output some useful crowd control, but keeping his damage mostly indiscriminate forces him to be less powerful than he could be, as he ends up "wasting" a lot of his strength on collateral damage. Add the fact that he's not supposed to be good at waveclearing, and that he's an assassin who happens to have a lot of bruiser-friendly anti-spell measures (i.e. his magic damage resistance, his shield), and the end result is a champion with quite a few identity issues.
To solve this, I want to dissociate Kass's damage from his base spells, and make sure that he can still afford to have some measure of team CC without it bleeding into waveclear or an AoE nuke. Additionally, the lockout mechanic means that he can afford to have strengths that would normally be excessive on a single champion: as a relatively CC and utility-heavy assassin, Kassadin shouldn't really have good target access, which his mobility gives him to some degree, but the lockout mechanic means he needs to commit to an opponent, which enemies can play around as well. As a result, he should be able to pursue single targets well while still being able to output powerful non-damage effects for his team.
Beyond kit changes, I didn't change Kass's base stats, other than the removal of his mana. Mana is nowhere near as universal a resource as it's made up to be, and in Kass's case, it's really not an adequate gate to his spells or mobility. I think Kass could afford to be manaless if his kit gave his opponents more opportunity for interaction, since his live Q and E especially would be abusive without a mana limitation.
COOLDOWN: 9 / 8 / 7 / 6 / 5
Active:Kassadin projects an orb of void energy in the target direction, which remains in place for 1 second before launching itself forward, stopping at the first enemy champion hit.
While Null Sphere is stationary, all enemy projectiles that pass near it are halted mid-air, resuming their original course once it begins moving.
Null Sphere has always caused issues due to its point-and-click nature: as a point-and-click silence, it was a horrible anti-counterplay nuke that completely prevented enemies from reacting properly to Kassadin as he jumped on top of them, and as a point-and-click interrupt it's a mostly bland ability that only sometimes becomes more interesting when against certain champions. Its shield is also in a weird state where it can't be too strong, lest it turns Kass into a bruiser, but can't really go away without making him really suffer in lane, and I think having Null Sphere pause projectiles would allow Kass to outplay mages on a much more subtle, yet potentially far more effective level. It wouldn't be as good for poking, but I also think his E should cover that function as an offensive spell.
Active:Kassadin's next basic attack gains bonus range and slows his target for 1 second.
I moved Kassadin's slow to his W for a couple of reasons: I want to try giving him back his silence, which I'm moving onto his E, and I also think Kass's sticking power would be better-suited to a melee effect. With this, he'd be able to bap his opponents a lot more consistently, especially since his innate would give him the on-hit damage he'd need against champions, which could help make up in part for the reduced reliability on his other effects.
Active:Kassadin fires a pulse that enters the first enemy it hits. If the target was a champion, they erupt after a delay, temporarily creating an anti-magic zone around them at the time of eruption.
Zone Duration: 1 / 1.25 / 1.5 / 1.75 / 2
Enemies with their Spell Traces locked are silenced while in the anti-magic zone.
In its current form, Force Pulse doesn't really fit on Kassadin's kit: he's an assassin focused on hunting down individual targets, yet the ability's an area of effect, and its AoE damage clashes with his intended weakness of poor waveclear. On top of that, it has two different time gates that add little to the spell's gameplay, especially since its application doesn't carry much interaction either.
If there's one thing interesting about it, though, it's its AoE crowd control, and makes Kassadin excellent at disrupting crowds with a surprise mid-team slow. I think it's worth playing up his anti-magic theme here more, and change the effect into a massive AoE silence: the interaction with the innate is there so that Kassadin gets to damage his main target, but give them plenty of room to outplay him (something his old silence didn't achieve at all), whereas the AoE CC would give him some breathing room while he tries to kill his target in the middle of the enemy team. Effectively, this silence is meant to be somewhat of an equalizer, so that Kassadin can briefly try to kill his target as if he were dueling them, rather than the oppressive (yet iconic) disable that he used to have.
COST: 1 Charge
COOLDOWN: 6 / 4 / 2
Passive:Kassadin periodically stores a charge of Riftwalk, and stores an additional charge upon consuming a Spell Trace, up to a maximum of 5 stored at once.
Riftwalk is a tremendously powerful ability, making Kassadin one of the most mobile champions in the game, yet it's currently also gated in a really awkward manner: because its use is tied to mana, Kassadin can render himself a sitting duck just by overextending on his mobility, and ends up being pushed to itemize for mana in order to become more mobile, which puts him at risk of building tanky. Kassadin needs some limitations in order to not simply out-blink every other champion in the game, but his ult needs to be gated differently.
To solve this, I think it might be worth tying Kass's mobility to his innate: outside of combat, he'd have a limited amount of charges to work with, but once he starts fighting, he'd get to build them back up more quickly, which would allow him to excel at hit-and-run plays. It could also potentially introduce some better risk-versus-reward planning, since Kass could try expending all of his charges in order to roam faster, and then recharge them quickly by focusing his target of choice, or play safer and have more backup mobility in case things go sour.
What are your thoughts? Is this what you'd like Kassadin to be? Do you think this is a good assassin kit, and does it fit Kass's playstyle?