Best way to change fizz.
Warning: Joke alert. Do not get your jimmies rustled.
What is Fizz's ult had a global range? Almost nothing changes about it besides even further scaling on the damage. Missile speed is the same, hitbox is the same, shark size doesn't get any bigger, just the damage revs up to ludicrous amounts if it's traveled a decent distance. I would play this.
Make the rest of his kit plain, and just add in this ultimate. Throw sharks from half a map away that will one shot if someone gets caught by it. Enemy team doing baron? Shark that thing from fountain and watch it deal like 1.5k base damage with a 300% scaling.
Without joking I'd love to actually see a champion like this. Global range nuke ult that's hard to land but is game changing or atleast will guarantee someone dies. Basically Jhin's ult in while he was in development. That my friends, is a real sniping class champion.