Here is a list of proposed changes to flesh out the Holy Priest spec in PvE:
Serendipity: make it also allow Prayer of Healing to be cast on any party in the raid like in WotLK onward. This would complete the Holy Priest synergetic toolkit around the Surge of Light and Binding Heal runes since the latter adds 2 stacks each cast.
Spirit of the Redeemer: Increase the duration from 10s to 12s (or 15s like in TBC onward). Potentially add a bonus 10% to 20% healing done over the duration. This would make it actually compete with Dispersion on every single boss fight and Discipline Priests can’t really use it because PW:S cannot be cast while transformed.
Binding Heal: Buff this rune. Either add some new effect (e.g. Empowered Healing or Holy Concentration talent from TBC onward?) or outright increase its healing power coefficient/reduce its mana cost. As it stands, it only heals the target for 333 more than max rank Flash Heal on average for roughly the same mana cost regardless of healing power. The self-heal component is not really relevant because most raid groups have a Shadow Priest in the healer party for mana regen and Vampiric Embrace keeps the healers topped off.
It’s not too late to design two distinct healing playstyles for the only class with two healing specializations!