Hello

You are asking constructive idea, well I agree with this post, but I'm gonna try to be realistic,

You can not ask "skill <X> be able to use during cast time" There is no one skill of that kind right now in game, maybe for design issues.

My idea to fix current state of BRD/MCH:

WM/GB cast timers reduced to 1 second casts.

Why?

Current issues are based on oGCD skills, I mean, If I have 2,5 seconds of GCD, and skills cast take 1,5 seconds, 1 left for some oGCD skills like Reload, Misery's End etc...but, a lot of oGCD skills have more than 1 seconds of animation lock, between 1,5 - 2 seconds and this fact makes BRD and MCH to spend between 0.5 - 1 second of the next GCD...this can sound a bit ridiculous..."OMG, 0.5 or 1 second..." but during a 10 minutes encounter You could be losing a lot of GCDs for a desing issue.