Take a hint Riot from Pokemon

Instead of introducing Keystone Masteries that break the game for some champions. I am pretty sure that CotC was NOT designed with in mind as it gives her two shields essentially.

I agree that it is totally cool to have customization but I feel that runes and masteries are just NOT the way to be doing this. Instead of providing flat power to everything with no cost, why not introduce a system like "Pokemon" has where you can customize small increases of base stats to the detriment of others. (IV and EV Training)

This means that a could choose to customise his base attack but at the EXPENSE of his defense. Giving a clear trade off that allows people to find the stats that work best for their individual playstyles while at the same time, providing a double edge sword that weakens a champion in another area.

The pure problem with the current system is that the mastery tree gives a metric ton of added damage and other abilities with no detrimental cost to balance them.

If you absolutely HAVE to have "keystones", then make them available at the end of a tree so that you pay a significant price for actually getting this keystone.

Allowing people to customize small increases to their base stats directly while allowing them to have negative returns as a trade off for the additional stat bonuses would be a far more effective solution in my opinion. I won't pretend to be a "Stat master" but i think the lack of trade off is whats causing the game to become stale.