If the boss/enemy fight mechanics design ppl are pushing towards a direction which I think they're going for, then I don't think that, by the time they're finished with some of the later boss fights, that any tank will have ANY time at all worrying about their block/parry cd's when they will be bombarded with tasks like positioning, enmity generation combos, etc...

Why?

Because they're currently designing fights WITHOUT taking into account that you need to press a button to activate block/parry. Also, the balancing of stats will also change with a system change like that.

Not saying that your idea is bad, but I'm not sure whether the dev team has already finalized overall outlook of the battle system and how it'll merge together with future fights, item stat distribution, and character customization options. You have to consider all of these together when changing designs like these.

Just my 2c... Cheers...