This buff will not address Fiora's fundamental problems. Her problems are that she has no true escapes, has no ranged farming tools, can't duel bruisers head on, over reliance on her ultimate for power, her squishyness, and doesn't have cc. This suggestion of blocking on-hit effects has been asked by Fiora players since she's been released but it won't make her much more viable. She still has too many weaknesses and not enough strengths to make up for it.
Secondly, does this buff include on-hit effects that convert physical damage into magical damage such as Twisted Fate's W or Poppy's Q?
Thirdly, are you going to adjust her passive? It is comparatively awful compared to Xin's W, Irelia's W, Warwick's passive and almost every other sustain passive out there. Fiora is supposed to be able to duel tankier opponents in laning phase by using Q + AA + Q + AA and retreating to let her passive heal her to win the trade but the numbers aren't high enough to support your design intent. It then becomes completely negligible come teamfights. Her "duelist" passive ironically doesn't help her in duels.
Lastly, what is your overall intent for the melee AD class? Right now they are a essentially a soft assassin class with less burst and more cleanup power. Fiora's best role in teamfights is isolating the ranged AD carry and assassinating them as she would simply get cc'ed and bursted against mages and can't outduel bruisers. Is this what you want for Fiora? An assassin with minimal disengage? You can't play her is the only AD carry on the team because she has no poke, can't siege towers, and has to wait for clean up duty. Melee AD carries like Yi, Tryndamere and Fiora are too reliant on their ultimates for power, which makes them hard to balance.
I've thought a lot about Fiora's design and problems and I'm glad to see Riot is finally looking at her. I just don't want it to be a band aid fix and then have her ignored again.