Camille's ult would better fit Fiora to be honest

When she was being reworked, I thought Fiora would get such ult.

She'd target enemy champion, knock away any unit, enemy or allied, except for herself and the target and would create zone where no ally or enemy can go or affect it with ability and simultaneously zone she or the enemy champion now called Challenged Enemy couldn't eave. Fiora, being in her element, would get bonus Movement Speed and Attack Speed. After several seconds, the area would disappear and they'd be free. If Fiora kills the target within the time, the area disappears prematurely and Fiora keeps the MS buff for several seconds as well as gaining shield. Also, she'd get stack of Fame, stacking until the end of the game infinitely (like Nasus or Kindred stacks), granting her a little bit of either AD or Armor and Magic Resist. So the more enemies Fiora challeges in the right moment, the more Fame she gains.

Only problem would be this ult would have little explicit non-abusable niche. It could avoid being abused as disengage since Fiora traps herself, too. However, it should be ult for spliting dangerous enemy from her team and taking on them herself. Meanwhile, it would get abused to reap low-hp guaranteed kills for free AD or ARM/MR.

But you know, it'd fit her. She's literally the Grand Duelist.