I love Lux. Lux is a lot of fun to play. When I first started out she was one of my first unlocks just because her kit descriptions sounded like so much fun. I love her utility I love her personality and I love her fun-factor.
But she's very weak right now, and I think her passive is part of the problem.
Lux's on-hit passive would work better if it was more like Zigg's. Ziggs has a very similar passive to Lux, except his works on towers. But Zigg's passive gives him one very strong hit followed by a cooldown on the effect. Lux gets a weaker on-hit proc but she can spam hers with multiple rapid spellcasts.
That makes Lux's sound better on paper, but in practice it slows down Lux's burst damage and her combo. To maximize her burst damage Lux has to Q, auto, E, and auto again. If she chains QE into one damage burst she misses out on the damage of a Passive proc between them. Her Ult has a really good perk of both detonating and then re-applying her passive, but with the massive range on it how often can you feasibly USE that second proc?
Damaging the enemy too quickly COSTS Lux damage by way of wasted passive procs. But stopping to weave auto-attacks in between each spell gives her opponents a large amount of safe room to escape Lux's E before she can detonate it or evade out of auto range entirely and deny procs.
Personally I think the best thing to help Lux would be to shift damage from her passive into somewhere else on her kit and give her a utility passive.
Buffing R's damage would probably be the straightforward choice since it detonates the passive anyway, but adding more raw damage onto such a long range ult would probably cause frustration issues. What I think would work would be to shift the damage from her passive onto W so it damages enemies it contacts, but make it no longer automatically return to Lux for a second shield application. Instead it returns in a line shot and Lux has to manually intercept it in order to be shielded a second time. For a new passive, since Lux already has the power to reveal enemies and grant vision with her skills, Lux could be granted a temporary damage buff against revealed enemies. E or R, reveal someone's presence, and they take extra damage as punishment.