Also while this may be champ design difference:
Why does Fiora's W while:
- not being a gapcloser
- not becoming invincible against turret damage as opposed to Fizz's total invincibility
- covering smaller area
- dealing potentionally less damage (counting Fizz's Ultimate amplification)
- slowing only the first enemy champion, while having shorter duration [have to note this is still poor tradeoff for additionally slowing AS]
[- having weaker slow at rank 5, saying this to note it but I don't count it, Fizz's is scaling while Fiora's is not, Fizz's better later, Fiora's earlier]
have significantly bigger CD at any rank than Fizz's E?
The only thing where Fiora is better is she can stun one target, but that needs to have the enemy not observing enough, Fiora with good reflexes and champions with obvious and delayed movement disable, WHO STANDS IN THE LINE, which usually are only melees. The stun isn't that easy to trigger and I don't find it to be good enough reason to rise the CD for 10 seconds difference.
DISCLAIMER: This post isn't meant to be asking to buff Fiora, just to show how she pays for fcked up invinciblity from W as opposed to how Fizz "pays".