So umm...why not just rework Fiora if you think her R provides no counter-play?
Tiamat lets her turn an already low-counterplay ability (Blade Waltz) into a high-damage area-effect low-counterplay ability.
I mean, she's a bit of a failure in more than just her low-counterplay R anyways, so why aren't you reworking her instead?
For instance, besides a sexy French accent, her sole-defining characteristic is that she fights with the most undignified and girly sword out there. Yet nothing about her kit really reflects her choice of weapon.
Her Riposte doesn't reflect what it should be. She doesn't parry. And in the end her kit could have been given to literally any sword-wielding ADC. She has a Passive that suits a bruiser or tank far more than the girliest class of swordsman, and worse, as powerful as it can be, you never even know whether it's helping her or not.
So why not just give her an entire kit overhaul than just nerfing her R?