@Meddler An unpopular request for an unpopular guy... I think you should buff Kassadin.
I'm gonna start off by saying I'm no Kassadin player. I honestly don't even particularly like him. I have very little understanding of his mechanical workings, and if I were to get in a game with him right now, I'd probably feed. Now that that's out of the way...
Kassadin as a whole, it not an easily balanced champion. In terms of how efficient an assassin he can be, his ult alone would allow him to be one of the best. Most of us remember the horror of Kassadin in his prime, the Voidwalker who turned Summoners Rift into Swiss cheese the way he was walking in and out of it. Riot gave him the ol' back-alley nerfing in the version of a mini update, and we haven't heard from him since. The thing is though, I think in this particular season he could manage to do really well. I tried him a few times in customs and normals though, with little success. Part of it is his skill floor, sure, but it felt more than that. As he is now, he's a late-game champion, needing a good few items to really get moving.
The thing is though, as I played him, early game he didn't really have the safety net other late-oriented champions and assassins do. Riot made damn sure to limit his mobility until the game is gone on much later, which is fine, but in turn he's somewhat lost the thing he relied on to get him through the tough match-ups and lane harass. 
for example relies on all his dashing around and untargetability to win fights, making him hard to hit. 
has fall back patterns if she gets behind/low on health, and is mana-less, so it's harder to push her out. Hell, even 
can stealth for a time. 
, no, he's gotta take all the hits like a man with that puny shield until he hits 6, and even then his ult is on a 4 second cd, which is plenty of time for his opponent to retaliate if he uses it offensively. Even past the laning phase, he's barred from having assassin level mobility until you finally max his ult at 16, making it much more difficult for him to do his job if he can get caught so easily. So where's Kassadin's safety net?
Here's what I propose. His W, Nether Blade, already has pretty hefty mana sustain, especially if it hits an enemy champion. What if it allowed him to top off his health bar too? I suggest giving his W the extra effect of healing him for a very small flat amount. If he uses it on an enemy champion, the amount is tripled plus a % of his missing health. The regular heal is so small he can't abuse it in lane, gives him in combat sustain to keep him alive against EVERYONE (not just mages) for when his damage isn't enough to kill a target in the first combo.
