Everyone knows Fiora gets shut down by CC, so I think it would be cool to give her a counter CC. It would go well with the idea of a parry, since after parrying it would leave your opponent defenseless. It would also give her an escape mechanism, just pop W when you get ganked, and allows her to disengage in trades, so she doesn't have to commit so hard anymore as a melee champion. I think this would make her more viable, and it's a reasonable buff. Also, the fact that Riposte does not block on-hit effects like a slow or Teemo poison would be countered by the stun, since it would at least give her time for the effects to wear off.

The cooldown and/or mana cost can be raised to compensate so that Riposte can't be spammed, since no one will want to engage her if they know they're going to be stunned.

What do you guys think?

i personally think that her entire kit itself needs to be reworked, but this sounds like it can be done with just a few lines of code lol

NO

yes lets take literally the ideal champ for 1v1 and give her something that will make her 1v1 even better.
however i do agree that she is broken. but in this state she is great for cleanup fighting in Pub games

not a stun, just a slow. I feel that Fiora's main problem is not being able to really stick to someone even with her MS increase.

absolutely no. and you're wrong adding a stun to her riposte will take a lot more than a few lines of code