User blog comment:LivesByProxy/Mastery Redux & Game Design Theory/@comment-1330314-20160120070154

Though I'm personally not a fan of the mastery system as a form of out-of-game power, I like the idea behind this rework, as well as a lot of the masteries you propose. On a design level, I completely agree with you that masteries, should they exist, should encourage both interesting choice and higher-quality gameplay, and while the current system somewhat achieves the latter, it doesn't offer much of the former (and not just because Thunderlord's is stronger than every other Keystone right now).

My main criticism here, aside from the balance being off on some of the masteries you propose (Agility looks really weak for what will usually be a once-per-fight bonus, Flanking and Numbers look way too strong), is that your masteries are often each doing several things at once when they could be concentrated into more impactful effects. Most of the current Keystone masteries offer a specific bonus with a clear gameplay goal in mind, whereas masteries like Inspiration and Architecture may have a single theme, but do too many things at once (e.g. Inspiration's Tenacity aura, Architecture offering both better tower offense and defense). If there's one thing the live Keystones do better than yours, it's that they're completely built around visibility, whereas many of the effects you're proposing don't sound like they'd give the same immediate visual feedback as, say, Thunderlord's or Warlord's Bloodlust, and concentrating your effects into narrower, but more powerful bonuses could help increase their visibility.

A more minor concern I have is how you're going to map mastery progression onto the current summoner leveling system, if at all, but I'd be all for a mastery system that doesn't force a hard barrier of entry onto newcomers.