With the new rune system revealed
How long will it take to go through its life cycle? I'm going to be a bit of a kill joy. Yes it's new, it's going to be interesting for a few months at least, but looking ahead a year or two from now, left to it's own devices (e.g. riot not adding in a new path or adding new runes in the existing paths), it seems like it will be in a far more exaggerated state of stagnation than the current runes and mastery system.
The cookie cutter mobafire picks will still be a thing and the exaggerated differences between the champion/keystone combinations will arguably be more noticeable, because the benefits have become greater.
They've pretty much laid out for you which classes are allowed to use which path and it's pretty easy to put any champion down a path based exclusively on their classes (ignoring their identity), even what's likely to be their optimal path. There isn't much room for experimentation, because you see a much faster drop off in power when you don't go into the appropriate paths. You pretty much just get what you see with it.
On top of that, champions are going to be even more constrained to being designed to make appropriate use of at least one of the keystones, because failing to do so would be a devastating power loss, but again to a greater degree. Just as it is with the current keystones and why thunderlords has such an easy condition and is the go to keystone (which seems to be getting replaced with Summon Aerie).
I'm also a bit concerned with how many of the mechanics are designed to stack in effectiveness with champion take downs or just permanently, because these kind of mechanics have always resulted in more snowballing. As if the game doesn't have enough already.
The changes will absolutely be a breath of fresh air for the game in the short term, but i think it's going to only exaggerate the problems that it proposes to address in the long term.
When the problem is stagnation and flexibility, a new system which attempts to address these problems will always feel like it's an improvement, because it's new and untested. Though looking at the actual design of the different expressions of power, I don't see this making any improvements to the old system which diffused the power effectively with very generalized increases.