Even before the new masteries, if you checked -- Lucian had been a top pick for a long time. Including when he sucked.
As it's clear though, Thunderlords on Lucian is strong early for Lucians who know how to poke. If Thunderlords only gave 1 stack from his passive, Lucian wouldn't be as oppressive (even though I personally have not ever felt oppressed by Lucian past his release and Fervor giving him more teamfight over oppression in lane). But I don't see any need to nerf Lucian considering he's not unhealthy -- it's more telegraphed with his position and your minions than Miss Fortune's Q (which is more powerful) poke in lane as well as insta Thunderlords E and cc from range. Lucian has mobility, but only the experienced players can really utilize his W very well. If there is a nerf to high end players that doesn't hurt those who don't main Lucian but still like him, it might be nice. But as is, just not problematic. He doesn't have any bullshit in his kit other than his high burst if ahead, but he has the tiny range to compensate for using a full burst on a target. He has clear strengths and weaknesses of which are exploitable.
But you also brought the point of diversity. I re-iterate Lucian has almost always been a top pick in Solo Queue even when bad. Notice he's not too popular in Competitive anyway. It would be very awkward to nerf him into uselessness to bring other ADC's in when it's shown his power has a minor effect on his pick. On that note, I don't know what would bring more diversity in ADC's other than nerfing every ADC slightly to have more niche, or to buff existing niches. (Also considering Morde was hated, overpowered, underpowered, and then scrapped, what really is diversity?)