I see four huge design problems with this idea. First, by giving them the shield when they are hit by CC, you are guaranteeing that they'll have the shield at the time they need it most. With the current version of the keystone, someone who gets CC'd can back off and throw a retaliation out once their enemy's shield dissipates. However, with this change, you're guaranteed to at least block the damage of the incoming CC, and you're giving them a shield for the full duration of the CC, which is when they should be vulnerable. The attacker has no options except to brute force through the shield.
Second, this would make it the only mastery that are forced to not pick based solely on the enemy team comp. There are champions like Nautilus that really don't want any other masteries than this one (Grasp feels fucking anemic in comparison for a Nautilus), but you'd basically force them into picks they don't want because the enemy team doesn't allow for them. That would feel sooo fucking bad. I do not want to be forced into taking an inferior choice because the mastery won't work with my own kit.
Third, by making it trigger based on an enemy's action, you make it either completely useless in lane, or a total hoser mastery. Taking it against someone without hard CC (like Rumble) would feel terrible, even if the rest of the enemy team is loaded to the teeth with hard CC to enable your pick. However, if I take this against someone who applies hard CC as part of their standard combo (Sion, Keld's Q, Riven), then I'm basically guaranteed a shield every time I trade against them. I don't need to be a tank to like that. I don't need to be defensive. I just need to want to win trades. This means that the most important thing determining your keystone choice wouldn't be what champion you're playing, or what playstyle you want, but who your lane opponent is.
Finally, by making it completely unreliant on any aspect of your own kit, you're opening it up to anyone to take. If you think Riven and Yasuo are annoying with it, imagine when any ADC facing a dive comp can take this for free barrier that pops up whenever they're in danger. It would be a hell of a lot better than running warlord's on quite a few carries. Or hell, just keep thinking about Riven and Yasuo. No reason any divers would feel bad about a shield that's basically guaranteed to proc whenever they're diving a team with any decent amount of CC. Only now, you'd also have to balance it around all the fighters who don't have CC. You'd have to balance it around every champion class being able to use it, or you'd have to restrict it to tanks-only by making it a bonus-health-only shield (and even if we wanted to do that, having it work off of aggression is still a better design because of the first two reasons).
Keystones are meant to be an enhancement to a champion's playstyle, not a response to the enemy's playstyle. They work best that way. Changing them to not be based on a champion's playstyle is a bad move.
Then the amount of shield is always a limited value.( only 1 CC proc CoC in same time)
I don't know what you're trying to say here. Why would it matter if only one CC procs CoC? They're still getting the same shield value if they're surrounded by enemy champions. How would this limit shield amount?
Just to make sure we're on the same page, are you aware the the number of champions someone hits with their CC is completely unrelated to the shield they get?
, which make it hard to balance.
diving my adc, do I cc him and give a giant shield or just hope my adc can kite it out?