Problem with adding a slow to Gravity is that slow has diminishing returns like stun does, and that wouldn't play well with Tank's Arm's Length.

You could make it so it has a 5 sec slow that doesn't share the DR timer or add a blind to it (since blind was removed from Flash a Long time ago).

But I do agree, SCH/AST do need some sort of debuff attached to their AOEs to make them even remotely equal weight to Holy.

IMHO though, I don't see the stun from Holy surviving into the next expansion. SE has decided to nerf healer utility in expansions, not increase it.