So we know the 4th assassin in the Big 4 for the assassin rework is AP..,
So let's go and figure out how Riot is going to change Fizz. Honestly, unless by some miracle (for me since I'm a satanist and love Fizz) the last big 4 assassin is Akali, we all can assume it's Fizz. So, in the comments, people should say what they think about Fizz (and Akali just in case) and how they would rework them without ruining their identity.
For me:
: As someone with over 100,000 points on Fizz (I know that's not much, but it's my highest out of all my champions) I love Fizz's kit. You don't have to be a genius to know that most people put Fizz on the same level as Teemo as being irritating to play against or simply being Satan. Their are two clear reasons for this, the first being his W and the second being his E.
Starting with his W, I like Fizz's W. The feeling of watching the enemy burn is so satisfying, especially when the enemy flashes and still dies. However, his W doesn't really make sense in his kit, or at least the DoT portion. Rendering you try to keep his (once upon a time until people started building luden's) build of Lich Bane Zhonyas Morellonomicon, or at least keep Lich Bane as a main part of his kit, I do think that his W should amplify his auto attack damage, or at least his Q damage. Since Fizz doesn't have an escape when his E is on cooldown it feels good to be able to keep doing damage in fights. Additionally, his active W is really helpful at csing under tower since generally Fizz gets pushed in by pretty much everyone early and is generally forced to cs under tower. If you feel like Fizz's W doesn't have a form where it's fair to play against while feeling good as Fizz, another damaging ability in conjunction with probable nerfs to his other abilties or a spell that is more defensive (rendering his E gets a change to be less defensive) would also be good if he stays as a diving assassin who generally is in the middle of a fight trying to kill people.
Moving on to his E, truthfully I think his E is in a good place and moreso needs better indicators on it, When I say this, I'm talking about the damage area. I can't tell how many times people have gotten away even though I landed on top of them with my E just because it didn't do damage or how I can be not close enough to someone when I E to hit them, but somehow it still hits them, slows them, and inevitably gets them killed. In short, I think that his E should have an indicator sort of like Lux E that shows the hitbox so not only the fizz player knows what the hitbox is but the enemy does too, or maybe making it he when he jumps on the pole it shows the range in which he could jump to the enemy so they can get used to knowing how far away to be to not get an E to the face. I'm sure if you (Riot) save his E you will have some better idea than I had to balance it, this is just my two cents.
Next, his passive. Personally, as much as even Riot thinks Fizz's passive needs work, I do think it should stay on his kit, even if it's like Graves' True Grit that was moved to his E. Ignoring unit collision just seems very important on his kit considering how much he's diving on top of people that could body block him. Additionally, his natural damage reduction from autos is crucial in the early game as generally he takes autos from just about every ranged champion in lane and would get bullied out by level 3 without it. If you were to look for a new passive, I don't really know. I'm just worried that he could end up being useless until level 6 (or even the end of laning phase barring roams) if he had no way to sustain himself in his early levels.
His Q to me feels like it could be a good spell if it weren't a slave to his W. The base damage and scaling on his Q are abysmal and if his W were to change, a simple buff to the damage would suffice in keeping it relevant, or a nerf depending on how big the W change is.
His R, like his Q, doesn't really need much, but I do think you (Riot) should look at that hitbox. The latch on latches on when it shouldn't and doesn't latch on when it should A LOT. Also, I don't know if this still exists, but I remember near the end of season 5, I can recall at least two moments where a champion would go invulnerable (Zhonyas), I would ult where they were standing in Zhonyas, and then as they got out of Zhonyas they would be able to dash away without the ult latching on if they were fast enough. This happened as I recall with a Katarina and a LeBlanc E and W (respectively).
: If somehow Akali is the last one and not Fizz, I can't input anywhere near as many ideas as with Fizz, but I can say maybe making her ult sort of like Diana's ult where when Akali marks someone with her Q and ults them, it refreshes the cooldown (or reduces it) as it would make her first rely on using Q on people, and second would make her less hit level 6 and be able to stick on to anyone with 3 ult stacks.
I truly hope someone found this useful and I would love to hear your ideas, not that I can make them happen, but to give Riot ideas on what to do.
P.S. I didn't think it would be Ekko due to his kit being new and unique or Kat as she is League's reset queen, but feel free to talk about them too.
P.S.S. LeBlanc isn't here due to being the third of the four "Big 4" (supposedly).

(some consider him an assassin)
