The easiest way to make Lux balanced again....
Would be to revert the 5.17 buff (Lux's Q has a lowered mana cost and now binds both targets for the full duration).
This way, Lux's Q not only returns some of the complexity and counterplay it had before (stand behind minions), and lowers her powerlevel somewhat without breaking her into pieces like what happened to Veigar. The slightly increased mana cost will also make her a little harder to spam her skills willy nilly.
I love Lux, and she's my girlfriend's favorite champ, but statistics don't lie: she's a bit too strong right now.