LeBlanc Problems (With math)
I stayed under tower all of laning phase as Viktor, using E to clear lanes and still ended up feeding to Mrs. Damage-then-back-to-safety. 
Fizz was nerfed because he was toxic to play against with his mobility and damage. They're also the same since they have oppressive laning stages but are able to be shut down in teamfights, but that's not a proper way to handle laning against these champs.
LeBlanc can do what Fizz used to do but with her W, taking out 1/5 of your health with a single W and masteries (About 100 damage total), not even counting runes.
When Level 2 comes out she can do up to 210 damage with a single Q/W. (110 for double proc of Q, and W with masteries)
By that point, if you decided one potion was not worth using yet, that's around 300 damage or 3/5 of your health. That there is worth two potions worth of health. So, after using your pots and you're back up to full health, she is now level 3.
So with her full Q/W/E combo she is doing 100 from W, 110 from Q double proc and 90 with her E double proc. 300 damage total. At level three with Dorans, the average mage has around 680 health. They're left with 380 health, not counting any AA Leblanc is getting in. If she didn't decide to stick around, so you can't retaliate, not proc'ing her double E, that's 345, damage, without any AA.
You know have 18 seconds to decide. Do you wish to go back to tower at half health to waste money on pots, do you call in your friendly jungler to help, or do you stay under tower using some means of farm? This decision is given to you at half your health. Anyone else think that's oppressive?
So, what do we do with her?
We could make her more ult reliant, taking out some of her damage from her abilities into her ult? This would mean she'd have to at least wait till level 6, where either champ can be able to duel reliably.
We could increase her CD on her Q, since that is/was the problem with Kassadin, having an always up farm/poke tool. Make her decide whether it is worth used poking or farming or maxing first or last. It would also stop that annoying last minute Q with her double proc with E.
We could change her mechanics slightly, maybe making her W's aoe becoming more narrow, allowing people to sidestep her W like they do her E. Or making Q a skill shot, since a lot of her damage is very reliable.
TL;DR A majority of LeBlanc's damage early game is oppressive. She's the new Fizz in terms of dealing damage and getting out. Her damage output scaling from level 1 to 3 is 100/210/300. So, what do we do with her? Reduce early game damage and throwing it into her ult? Up her Q cd to make her choose which is important farm or poke reliability? Make her W, side-step able by making it Narrower, instead of a circle, more of an Oval? Or straight up turn Q into a skill shot.
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the list goes on, but leblanc doesn't have the extra something to do so. Ahri has waveclear in her q that makes it easy to open up an opportunity to cast her charm. Lux has insane waveclear in her E (i've cleared almost an entire minion wave with her E at only 2 ranks in it). Diana has the tankiness to be able to stay into that fight until she kills them. Leblanc has none of these, and before you say that she has her w for waveclear, it's not too great to have your only waveclear to be an ability with a 10+ sec cd and it being your only escape when you're a sitting duck without it because most of your damaging abilities rely on the proc/gap closer it gives you. I understand that people could just WQE in to deal the damage, but pre6, leblanc's entire combo won't be enough especially if you're itemizing against her/ have mr in your rune page. Also, along with skillshots, you get wonky hitboxes, like her E that is extremely unreilable due to it's humongous hitbox, it's impossible to weave between minions because more likely than not, it's going to get entangled in one. So I understand why people have so much trouble with leblanc, but a skillshot Q would just break her. Also undoubtedly Riot will just compensate buff her that will just make matters worse.
and the ability to outrun anyone with a 6 second flash. Her W has absolutely no way to miss, if ýou're in Q range and there's no way of missing Q.
and ward well (stop roaming), she'll be use less by mid game.