Fiora and Lux are still listed as each other's rivals. Here's an idea on how it could work.

Riot has never really explained a good reason as to why Lux and Fiora are classified as rivals. It can't just be a noble family feud thing, or Fiora would also consider Garen a rival (and she doesn't). This means that something specifically between the two Demacian ladies had to happen.

Here's my own theory, but take it with a grain of salt.

During one of her discrete missions, Lux uses her light magic to do a good deed, but Fiora spots her using her magic. Fiora later accosts Lux in private and blackmails her, wanting to use Lux's influence to help the Laurent family return to power. Lux is left desperately trying to find a way to turn the tables on Fiora.

Could this work, or is it too much politics?