A consistent problem Riot has had with their balancing methodology is that many of their changes are aimed at pro play, yet have a disproportionately large impact outside of it, often even more so at lower elos. The end result is a game environment where a lot of champions feel completely bent out of shape to the majority of players, simply because they were tuned around being played in a very specific manner in a very particular setting, neither of which are reflective of how said champions are typically played. Riot needs to take a deeper look into which mechanics affect which elos the most, so that we can get more targeted balance adjustments in the future, and hopefully fix champions in pro play without breaking them in solo queue, or vice versa.
One fear I have with the upcoming marksman itemization changes, in addition to the recent mage nerfs, is that it might create two entirely separate metas: in solo queue, nerfed mages will struggle, but AD assassins are very likely to become even stronger, because they'll have access to more upfront burst (i.e. Stormrazor), and more means of ignoring armor (i.e. Last Whisper's total armor pen). By contrast, in pro play, assassins are still likely to get shut down by organized teams, who tend to rely on CC to peel and lock down incoming attackers, rather than just raw defenses. If these changes aren't balanced right, we may end up with two entirely opposite metas, one where AD assassins risk dominating mid lane at most levels of play, only for mages to continue being the only viable mid class in LCS, because assassins still wouldn't be able to assassinate consistently there. Even if things don't get that bad, we're still likely to end up with a huge discrepancy in assassin performance between pro and standard play, which is going to make balancing the class even more difficult in the long run.