So, the first thing to think about in a rework for kassadin is his overall theme. Here is his dierect lore from http://gameinfo.na.leagueoflegends.com/en/game-info/champions/kassadin/
tl;dr: Kass is a boss guy who hates the void and anything assoiciated with it. He was at one time an adventurerer and was only able to survive his encounter with the void by letting the void inside him.
There is a place between dimensions and between worlds. To some it is known as the Outside, to others it is the Unknown. To most, however, it is called the Void. Despite its name, the Void is not an empty place, but rather the home of unspeakable things, horrors not meant for minds of men. Though such knowledge is lost in modern times, there are those who have unwittingly discovered what lies beyond, and they have been unable to turn away. Kassadin is such a creature. He was once a man forced to look upon the face of the Void and forever changed by what he saw. Once a seeker of forbidden knowledge, he discovered that what he sought was something else entirely. He is one of the few that has found his way to forgotten Icathia and lived to tell the tale, following the scant breadcrumbs hidden in ancient texts.
Within a decaying cyclopean city, Kassadin found secrets of the kind that he will never share - secrets that made him quake with fear at the visions of things to come that were thrust upon him. The power of the place threatened to consume him forever, but Kassadin took the only route available to him in order to survive - he let the Void inside him. Miraculously, he was able to overcome the alien urges that went with it, and he emerged as something more than human. Though a part of him died that day, he knows that he must protect Valoran from the things scratching at the door, waiting to get in and visit their torments upon the world. They are only one step away... something to which the appearance of the abomination known as Cho'Gath attests.
If you look upon the Void, you can't put it behind you. If you look upon Kassadin, he is probably already there.
so, here is what i think would be a bit more in with his theme. I think it is a good idea to keep his ult because there isnt anyone who doesnt like to xpeke, however this gives him an amazing amount of mobility. As a result, he needs to pay for something either in health or in damage. As an assasain, he shouldnt be lacking in damage, so he has to sacrafice his health. As a result, i thought to add a health cost to each of his abilities. This makes it so that one does not need to fear getting poked out before a fight starts, but does need to manage their spells well so that they dont die in a few hits. I dont know what kind of values would be very balanced, but i would love to hear them in the comments
PASSIVE: Void stone
Kassadin takes 15% reduced magic damage and ignores unit collision
This seems like a good passive for an assasain to me. It makes it so that he is to too op with his passive or reliant on it (like katarina), but at the same time he cannot get destroyes just by poke.
Q:
passive: Every auto attack restores x/x/x/x/x health
Active: Kassadin fires a skillshot (750 range) that damages all enemies hit. If the enemy is a champion or large monster, they are marked with traces of the void for 3 seconds. If kassadin attacks a target with trace of the void, he regains 15% of his max health (NOTE: Only 1 champion/large moster can be affected at a time)
This is a key skill for kassadin. It enables him to set up his engages so that he is not at as much risk. By tagging the enemy carry with this, he is able to safely jump in, burst them, and jump out without losing all his health. At the same time, this enables counterplay. Enemy tanks are able to jump in from, and tak the mark for themselves to protect their carries. Plus, a target that is marked will be able to know that the burst is coming.
W: (kinda like cho gath scream but about the size of kassadins regular e)
Active: Silences all enemies in an 60 degree cone for 1 second, and slows them by 30/40/50/55%.
damage should be low since it is more of a utility spell
Ok, so this sounds a bit op but hear me out. First of all, it would require a substantial health cost to work. But lets look at it in context. Your playing kassadin. You mark the enemy tristana, and jump in to burst her down. Without this ability, you are a sitting duck until you get riftwalk back up. So, this ability gives you 1 second to stay a bit safer. I decided to make it a 60 degree cone, however, because that allows counterplay. Say you are going in on the same tristana, the enemy braum can see that you have marked her and are probably going to go in on her and then gets between you and her. Now you have to choose whether you silence the team to avoid getting bursted, or silence braum to avoid the cc.
E:
toggle ability, upon activation it has a .5 second cd, but after that it has a 3 second cd until you can activate it again
Active: Grants kassadin 5/10/15/20% bonus ap, however roots him until it is toggled off.
So, this is a weirdish ability. This is where he sacrafices his mobility for more damage. Late game,when he is strong, this balances out his silence. It allows you to deal a whole lot more damage, but you trade mobility for it. Good kassadin players will jump in, silence the team, activate this to get their burst down, toggle it off, and then jump out. I think the counterplay is obvious here
Ult:
Well, this was kinda obvious. Same ultimate, but the mana cost is halfed and transfered into health cost (30/60/120/240/480)
Additionally, the range would be restored since the new kassadin needs some way to get out after the silence ends
Anyway, this was just an idea. Tell me how you like it or what you would change in the comments :)