Just an idea, but an interesting change to Lightspeed could be that instead of lowering potency, it decreases both cast time and the GCD (minimum 1 second GCD or 0.5 seconds?), but silences or "aether blocks" you from being able to cast any spells (Essential Dignity is omitted from this) for five seconds. So it's similar to Berserk; however, it can be cleansed by another healer OR be removed when you activate Luminous Aether.

Therefore the drawback from the current Lightspeed is removed, but a new drawback no healing for 5 seconds (or 3?) and faster stacking enmity!