Well that depends on what we want fizz to be. If the entirety of the Fizz champion is "can 1 shot you with ult up", then we've got a problem.
We could upgrade his push potential, make his E apply his W to minions so he can clear a little faster.
We could make his GigaShark (the max range one), more menacing, because you'd actually have to land the skillshot.
Lowering his Q cool down wouldn't be the worst, makes him more slippery and buffs damage over the course of a team fight, but not his burst (and is less of a 'get out of jail free' card than his E).
It's entirely possible to put the power in Fizz's kit into more reasonable mechanics. We have options.