If Lux weren't strong, would you upgrade her passive to give vision of affected enemies?
Lux is in a good spot, so Riot will not do this now, but imagine she weren't in a good spot. Do you think it'd be neat if she got an improvement to Illumination whereby it grants vision of affected enemies? It'd make sense on her given her theme.
Alternatively, it could be modified such that if Lux procs the passive by landing either a basic attack or by landing her ult, it deals damage and grants vision of the target for a limited time, maybe even with True Sight. That could be better if having the vision tied to the Illumination debuff is too overbearing.