Sure, we have a long CD equanimity & surecast...

Sure, too great of diminishing returns on CI would further break PVP.

However, I think this would be a welcome addition to PVP if implemented properly.

Perhaps every CI reduces your next spell's cast-time by 10% up to a maximum of 50% or something along those lines.

Or after so many you're CI-immune for your next spell with a cooldown on the immunity effect.