People complain that tanks are weak and honestly, I've made it known I strongly disagree. However, I will not deny for a second that keystones have made this game a nightmare to balance and have warped the games balance so many times this season.
And much of that is because reliable damage keystones like thunderlords and fervor can give literally anything the ability to do damage while not paying much for it in return.
People talk about tanks dying instantly, again I disagree but let's not forget that for half of this season, tanks were running grasp (which does pretty considerable damage honestly) or thunderlords and were running around killing everything while being incredibly hard to kill.
No matter what meta we are in, and I'd argue we are in a strange state where a little bit of everything is good right now, the amount of reliable damage any class can put out is absurd. And that kind of damage come from all classes is bad for the game, both balance wise and for just the sheer experience. Nobody enjoys playing against a champion that typically builds glass or semi glass and decides they will go full tank and abuse fervor/grasp procs. Nobody likes playing against semi glass or glass champions that build Trinity and tank after that and abuse Trinity procs + fervor. These are a few examples that illustrate my point.
In reality, the issue is this: runes and masteries should not be this game changing/warping. There's enough variables as is to consider when it comes to balancing and when you begin balancing around masteries that will eventually be changed anyway, you further set the game into a state of chaos. The old mastery system was fine, much like the rune system is fine now. Yes its stale but it has to be because otherwise you get this mess.
I urge you to consider removing keystones this coming season and simply go back to a more simplistic mastery system so we aren't spending chunks of a season running the same 2 mastery setups on every class until the next setup is buffed while the old one is being nerfed. And despite all of this and the attempt of making masteries more interesting, in reality the masteries with keystones have just become more boring. That tells you how flawed this idea is.