Fiora: Grand Challenge.
Fiora is..... obnoxious to say the least. Well, she's titled the Grand Duelist and her ult is named Grand Challenge, yet it's so one-sided and way strong dealing the max health % true damage that scales with AD that we all know and "love" (bear with me... I'm trying to not let the displeasure show too much)
So since she's such a duelist and is challenging someone, I think it should be fair if the enemy received similar opportunities when facing fiora. Her riposte already makes very little sense as it blocks just about everything and anything that any champion can throw at you....I'm not even sure how you parry a karthus ult really..
but onto the main point. Being the obsessive sounding person that she is, she should duel their opponent on more equal terms. Using her ult should also mark herself with the grand challenge and allow the enemy to deal % max health true damage to fiora if she's so confident that she is the best duelist and should also apply the aoe heal to the enemy upon victory (with varying scaling mechanics based on what the enemy is building) This should allow the enemy to counterplay an overconfident fiora who is actually unskilled while letting the actually skilled fioras retain their position by knowing how to fight properly during this situation and also knowing how to prevent the enemy from using it to their advantage.
earlier I was thinking of including the passive of having fiora randomly be marked with her own vitals but that would remove a bit of her uniqueness and would be, in my own opinion, a bit too easily exploited by ranged champs and possibly enforce ranged top picks that would throw things into a weird loop.... so I'm against that.