Here is a list of proposed changes based on many years of being a healing Priest main from Classic all the way to WotLK:
Binding Heal: remove this rune. It is rarely used in PvE content. It is mainly used in small-scale PvP content such as arenas, which are not in SoD.
Serendipity: make it also allow Prayer of Healing to be cast on any party in the raid. This would complete the Holy Priest synergetic toolkit around the Surge of Light rune.
Strength of Soul: Make the Rage/Righteous Fury generation portion baseline for the Priest class. It already feels unfun to have to pick or not pick an underwhelming rune based on what tank(s) you happen to have. Furthermore, with the new Soul Warding rune, periodically shielding DPS warriors without the Strength of Soul rune will gimp their Rage regeneration. This effectively reduces build diversity.
Renewed Hope: Somehow combine it with the Strength of Soul rune after the abovementioned change. They work similarly and are both individually underwhelming.
Soul Warding: Do not remove the CD of PW:S completely. This leads to the 1 button monkey gameplay from WotLK. Discovering tendonitis should not be the goal of this Season. Possible alternatives to the CD removal component include making Flash Heal, Greater Heal, and Penance reduce the cooldown of PW:S by 1s, adding a healing component to PW:S, and/or adding a Rapture-like component to PW:S.
Spirit of the Redeemer: Increase the duration from10s duration to 12s. Reduce the cooldown from 5 min to 2 or 3 min. Potentially add a bonus 20% healing done over the duration. This would make it actually compete with Dispersion on every single boss fight.
Finally, with the two additional rune slots that free up with the abovementioned changes, there are two possible options. Use both to either round off the existing toolkits OR give Holy DPS some love. With the addition of the 6% Holy spell hit Ring rune, making Holy DPS viable in raids with 2 extra runes should be feasible.
Side note: reconsider the approach of making all Priest DPS tier gear Shadow damage-only. Aside from killing the Holy DPS dream entirely, it also kills any future emerging hybrid heal/DPS builds. For instance, the Tier 1 Twilight Prophecy 2-set bonus that allows Flash Heal to be cast in Shadowform is incompatible with Shadow damage-only gear.