It is common knowledge that Kassidin is broken, and as such riot nerfed him to prevent him from disrupting ranked, and the LCS. At the state that he is in, you need to be very skilled and your lane opponent needs to make some big mistakes in order to make it past the the early-game, and actually be useful in the mid game. Mostly I wanted to remove his dependence on the Crystalline Flask (Rito's crutch item) and move him to a Doran's Ring.
However if Kassadin gets ahead, he becomes unreasonable to fight. He can show up and kill you and get out before you can lay a finger on him. The game is then dependant on building enough defense to survive him, after that you can ignore him outright. If everyone lasts until full build, Kassidin falls off hard.
During this I will focus on mechanics, numbers are subject to change.
How is he broken?
His pre-6 damage is unreliable. Null Sphere has sub-par range, and leaves you easily poked despite its shield. Other mid laners can easily hit him with their higher range spells while he moves closer and the shield only strong enough to shave some of the damage off of a spell. While this ability is supposed to help him trade, it doesn't do that job unless you are facing an enemy with a minor damage poke like Karthus or Katarina.
Force Pulse can only be used every 6 spells cast near him. While casting spells happens often in the mid lane, level 1-5 do not have enough spell casts in order to make this ability reliable. Nether Blade requires you to auto attack the champion in question. A champion with no pre 6 mobility does not get to utilize this in a lane filled with crowd control and/or mobility.
If he is not stomped in the early game he is either on par with the enemy or ahead. If you are ahead, you will be nearly unstoppable. Using Riftwalk you can chase enemies for days. When they are low their only choice is to run which will do nothing so long as he has some mana. When he is on par his options are limited. He is too squishy to dive into more than 1-2 champions.
If you DO get stomped, all you can do is act as a mobile ward that occasionally dies and gives the enemy team money.
These are just my observations.
We need to make his early game less punishing he needs to be able to contend with spam abilities and skill shot nukes alike. Champion with spam-able abilities often have built in weaknesses. For instance you can walk out of Karthus' lay waste before it goes off. Therefor I believe that his defenses should focus more on high damage abilities. Note that I am not attempting to make him imune to these abilities, just resistant.
I removed Kassadin's innate magic resistance and moved the shield into his passive from Null Sphere. The shield is much stronger to make up for the lack of magic resistance and how rarely it will be used. The largest change here is that any ability can trigger the shield, so you shield yourself on YOUR terms. His shield also scales less with ap so it won't be too oppressive based on itemization, but scales more with level. It is not reliant on ranks in Null Sphere. This will smooth out your defense throughout the game.
I put in a little quality of life feature. The passive can activate to your hearts content, it doesn't stack. But if you are shielded and trade with a champion it disables its ability to trigger for a while. This is to prevent it from being wasted while farming using abilities, or Riftwalking from place to place.
I wanted to keep an interaction between Kassadin's Null Sphere and his defense, so I replaced the shield with a direct interaction with his passive.
Mid to Late game
I nerfed the missing mana gained from Nether Blade on champion hits. This is his ability to sustain mana and kill things up close, and this mights seem like the biggest nerf in the history of Kassadin, but it will make sense when we get to Rift walk in the next section.
Also Force pulse needs to display the range at which it gains charges. Rito wants clarity, and this would be nice.
I won't put a poll here, as it just ties into the next one.
Ugh.. this... just.... Riftwalk...
So as I stated earlier, Riftwalk has several problems. Enemies are unable to escape as long as you have enough mana, and if you don't have enough mana, you are dead in the water. While managemet of your mana is important on any champion, the amount you have to deal with is astounding in this single ability. You must keep track of the stacks, damage, and mana cost. While doing that you need to keep track of your mana in comparison to the new mana cost, as well as the both teams placement, and the health of any possible targets.
Did I mention that the ability scales with mana for some reason? All of your abilities deal damage based on your ability power, except for this one?
I have several goals for my changes with this ability.
1. Shift damage to scale with ability power instead of mana. This feature is okay on Ryze because all of his abilities scale with mana. Singed has scaling health but you don't have to keep track of that in the same way you do with damage. Kassadin needs a clear difference between building resource management and damage.
2. Rework his constraint mechanic. Mana costs have too much of a long term detriment on him, forcing him to return to base. If he has build to handle this mechanic (Athene's unholy grail, and Archangel's Staff) he can ignore the constraint anyway, especially with blue buff. Either it kills you when you are under fed, or you ignore it when you are ahead on your items. That is the definition of broken in league of legends. I nerfed his mana sustain from nether blade in preparation for this change.
3. Add some visual changes that allow the enemy team to know more about the ability. Damage, availability, and direction are things that are unclear while just looking at Kassadin. Deception mechanics belong to Shaco, LeBlanc, and Wukong, not Kassadin.
These are some pretty big changes.
The stacking mechanic now has a name and winds down instead of just disappearing after 20 seconds. This makes spacing your Riftwalks more rewarding, and smooths out gameplay. However repeated uses of riftwalk build up different effects as you go along. They still increase damage, but now also increase cooldown instead of mana. This will make it harder and more costly for Kassadin to chase. Mana is still increased but only after 2 flux (third cast) it doesn't have any sort of compounding doubling effect. It doubles once and only once. The range decreases after 3 flux (fourth cast) this is where things really start to get hard for Kassadin. You can still jump walls, but you will have to get closer to them. After 4 flux (5th cast) Kassidin starts damaging himself. This will really force the player to conserve and wait for Riftwalks.
Ultimately these changes place emphasis on timing and placement, over itemization and mana management. Overall you will be leaping more, but it will be difficult to do so in short time frames. Chasing will still be a powerful tool that he will thrive at but it will no longer be hit or miss. Enemies will be able to predict how far he can chase.
I had to make one minor change: I increased Riftwalk's mana cost as it would no longer be doubling on every cast, but still needed to be costly.
Some visual changes I couldn't include in the text are small circle like particles around him that display his flux, a line like animation between his location and destination upon casting Riftwalk, and a static-like animation for when he is at max stacks, showing that more activation will deal damage to him. He does need another visual update, legs please???
This will change the way Kassadins build. They no longer have to deal with this strange dependence on mana stacking items. Cooldown may become a larger priority for them.
I do needs some feedback here though. I have put up a poll, but I also appreciate comments. I am not a professional game designer, and some of my ideas may not be the best.