I would like to argue that Fizz does not have meaningful weaknesses

Fizz, as we all know, is a total painintheass. He's maneuverable, does TONS OF DAMAGE, doesn't have trouble farming once he gets some AP (if he's willing to use Playful/Trickster on waves), is hard to catch, and snowballs rapidly. That and he acts as a counter to a variety of other picks, especially mages like Lux or Brand, due to his Troll Pole. He can even build quasi-tanky (i.e. Abyssal, Zhonya's, Rylai's) and STILL delete carries with ease.

Does this give him a lot of meaningful downsides? Not really. He's able to do loads of different sh!t that others simply can't do, and it generally takes more than one Champion to defeat him. The only Champion I feel he would struggle against is Malzahar, who is not a popular pick. Swain is also good against him (but also unpopular), while Syndra goes even with him (skill-based matchup, but she's got an early-game nerf incoming).

Riot, if Fizz is going to continue being in League, he needs to be given downsides that are MEANINGFUL, not TRIVIAL. Lacks durability? Trivial; he can build AP-tanky items to compensate. Exploitable cooldowns? Trivial; other more-balanced Champions have the same problem, and Fizz can delete you before you have a chance to exploit those cooldowns. Dodgeable ultimate? Trivial; he can delete you just fine with his basic abilities.

Here are some suggestions I have for possible Fizz nerfs (take them with a grain of salt):

1. Reduce Fizz's MR growth to 0. This would give him a similar weakness to Yasuo.
2. Reduce Fizz's health regeneration. This would make poke attacks stick to him and force him to rely on his Q and E to survive lane.

I am not saying I disagree with Fizz's firepower and maneuverability; they should stay. I feel, however, that Fizz needs to pay a bigger price for said damage and mobility.