I would personally change 3 things about AST, based on what I've been hearing and what some AST friends of mine have been describing.

1) Swap the 5% bonuses from Nocturnal and Diurnal Sect so that Nocturnal is 5% faster and Diurnal is 5% stronger.

2) Change Nocturnal so that the Aspected spells each apply a Healing Received buff, around 5-7% each, and allow the two to stack together.

3) Allow stancedancing between Diurnal and Nocturnal.

This would make AST be more of a support-focused healer, chaining their heal buffs with their regens to allow for the pair of healers to have even more DPS'ing time, and would further differentiate AST from the other two healers. Ideally this would make WHM/AST as powerful as WHM/SCH, along with SCH/AST, making it so that all the healer combos are about equal, while still being different from each other.