For me the biggest missed opportunity was the Marksman Update in Preseason 6. To me, that was the biggest chance Riot had given themselves to truly make some deep changes to the class, its itemization and its underlying mechanics, but aside from some good reworks, the class mostly just got powercreeped, keeping unhealthy mechanics like crit and lifesteal wholesale. The class became more positionally versatile, but did not gain any more interactivity (in fact, it lost some counterplay over time by having more burst), and its in-class diversity still remained unchanged to the point where Riot could still not concieve of possible subclasses to split marksmen into. It didn't feel like especially deep research was done into the class's core problems, let alone how to address them, and much like several other roster updates, the Marksman Update ended up feeling shallow, formulaic, and actually kind of problematic in the long run.
I think at this point though, League's competitive scene has become more prominent than it ever was before, and we got to see how damaging the stagnancy of the marksman+enchanter duo was to its entertainment value. On top of that, that same combo has had a major negative impact in normal play as well, to the point where I think Riot should hopefully be internalizing the message that we as players want to see much deeper changes to how teams are composed. So much work has been put into the game to add replay value, and I think at this point the issue of marksman-centric compositional stagnancy should stand out as a very high-value target. Even a move as brash as outright dumpstering the marksman role for a brief duration (which I don't at all advocate, as no player deserves to have their role ruined) could potentially provide tremendous insight into how much the game has been distorted around the class, and how it could evolve when the class no longer becomes essential to literally every team.
