Ever since preseason, Lucian has been on the rise as the jack-of-all, master-of-none of ADCs with a super strong laning phase and strong mid game, due to his synergy with S6 Essence Reaver.
In 6.7, Corki, who has a similar strength profile to Lucian, was nerfed out of viability with the removal of his passive's damage amplification (and honestly, good riddance). Meaning, Lucian's strength profile became even more attractive, due to a lack of similar alternatives.
Same goes for Ezreal, who took indirect nerfs from IBG's tank-based nerfs, and Kalista, who was nerfed directly.
What Riot will do with the nerfs is to give him a more pronounced niche, as well as toning down his general power level a peg. To that end, I strongly suspect that they will have a look at his mana costs.