@Riot : What is Xayah's individual niche meant to be ?
As much as Rakan has the identity of being the only Enchanter that relies on Mobility to get his job done, I don't know any particular reasons to pick her over others ADCs outside of having a synergy with Rakan, while he has arguments to be in a team comp without Xayah.
To be exact, her most distinct feature is her AoE damage and Waveclearing, but she has strong concurrence from Sivir, which has the same kind of AoE Damage and Waveclearing potential.
I am maybe missing something, but it seems to differ from the game design philosophy that you had until now.