I see 2 possible options:
Option 1 - put restitution talent way up into the holy tree. Right now I have to sacrifice either of my most powerful healing talents, i.e. Lightweaver and Divine Image, because it is on the very button. On the other hand right in the mid I have a dozen of talents, that are nothing but fillers and mediocre trash, which you could delete and I would not even notice the difference (Divine Service, Benediction, Renewed faith, to name but a few).
Option 2- make PvP talent Spirit of Redeemer a basic holy talent, making it accessable not just in pvp, but also in raids and dungeons. For those who do not do pvp: Spirit of Redeemer is 2 minute cooldown ability, that allows you to actively assume spirit of redemption form and get all the benefits and limitations (including immunity). After the buff is over, you get back to your ordinary form.
Blizzard apparently regards Spirit of Redemption as iconic ability that can save a dire situation and prevent a wipe. Being an active holy priest during season 2, 3 and 4 of DF (getting all the portals and going a bit higher), I find this ability to be canonically useless and most of the time it does not prevent, but delays wipe, which is even worse than having a quick wipe.
Of course, ideally I would like to have both of these options, but that runs a risk of making holy priest a meta spec in m+, which blizzard has been working really hard to prevent every season.