Ok, 2.0 since the first version got deleted
So, right now kassadins main problems are he is a high mobility assasain who ahs large burst. The underlying problem is that he has (or had) basically a free flash every 3 seconds. As awesome as this sounds, it does not allow for counterplay. Riot has tried a number of things from nerfing his damage, to lowering the range on his ult, but none of these really fix the problem. In the end, everyone wants to xpeke, and a lower range ult will allow you to do that. At the same time, nobody wants to play against someone who can come into your backline with almost no warning, kill everything, and get back out in just a few seconds. So, here is my re-work idea. I am not very good with statistics, so i would love to hear exact numbers (damage, cd, costs etc.) in the comments.
CHANGE #1: Kassadins abilities cost health, not mana.
Ok, so this is a pretty big change for any champ. But when you get a champion who requires health for their abilites and is still a mage, you usually get vladimir or mordekaiser. This would be a bit different though. The health cost balances his mobility. This makes it so that he can still jump into your team, but if he isnt smart he wont be able to do it with much health to begin with.
kassadin takes 15% reduced magic damage and ignores unit collision.
I personally feel that as useless as this seems, this is a pretty good passive. It can keep kassadin from getting poked out before he can jump in, but it also is not some crazy op passive coughpoppykatarinanasuscough.
Q: Lowish damage on initial cast, high damage if the mark pops. Kinda like LB
cost: x/x/x/x/x health
Damage: x/x/x/x/x +(x% ap)
PASSIVE: kassadins basic attacks restore 5/10/15/17/20 health
ACTIVE: kassadin fires a linear skillshot (750 range) that deals damage to all enemies hit. The first enemy champion or large monster to be hit will be marked. If kassadin attacks a marked target, he regains 15% of his missing health and deals (a good amount of) damage
So, this is a two part ability, first lets talk about the passive component. This is mainly for sustain in lane. This way, while he will still be affected, kassadin cant get pushed out of lane by his own health costs.
Active: This is where it gets fun. The ability is kinda like a mix between lb q and shyvana e. The initial damage is ok for poking in lane, but the mark is the important part. For kassadins benefit, this serves as a way to burst without getting popped. The health regen and damage are his main tools when assasinating a carry. However, because it is a skillshot it allows counterplay. Say kassadin is in a teamfight, and he sees a stray tristana firing from the backline. He can mark her with this, and then riftwalk in for a kill. However, the mark telegraphs some of kassadins movement. That means that if i am tristana, i know that its time for me to rocket jump away because kassadin is coming in. Additionally, if i am braum (tristanas support) i can try to block the mark myself, since it only affects the first enemy champioin struck
Damage: Low since this is more for utility
Active: Kassadin sends a pulse in a 60 degree cone that damages, silences and slows all enemies for 1 second.
So lets go back to our tristana kassadin scenario. You have marked tristana, and now you riftwalk in for the kill. But wait, YOU'RE IN THE MIDDLE OF THE TEAM! you already have reasonably low health because of your health costs, and now you are basically a sitting duck. This is what a dumb kassadin player does. The smart kassadin player can use this ability to etiher silence the team (to avoid damage) or silence tristana (to avoid her getting away) The point is that it requires desiscion making. Another important thing is the 60 degree cone, this is so that you can't be the op kassadin silencing the entire team.
TOGGLE ABILITY: This costs no health, and has a .5 second cd once toggled on, however if you toggle it off there is a 3 second cd before you can toggle it on again.
Toggle on: Grants +5/10/15/20/25% ap, but roots you while it is toggled on
Toggle off: Unroots, but you lose the bonus ap.
So this is an odd ability to most, but to me it is a necessary part. This is the ability that allows you to get your burst off at the right time. One of the big problems with old kassadin is that when he was good, people could burst wherever they wanted whenever they wanetd. When he was (or is) bad, people were too afraid to go into the team. This forces you to put yourself in a position where you can safely burst down the enemy. I think the counterplay here is obvious.
same old ulti, same range (before the nerf) costs 30/60/120/240/480 health using the stack system.