Holy did have HolyPower back in Cata and MoP. It was OK, but the need to get 3 HoPo for big heals made clutch healing harder.
I agree with the dps/healing idea. Legion was a step in the right direction, but it failed to capture the HotS-Uther playstyle. To me, Holy should be a spec that runs around swinging a hammer for 70% of the time and reads a holy book for the rest.
The "proximity = %healing" mechanic was alright, but would work better if it measured your proximity to enemies. The closer you are to mobs, the more healing you do. Holy could also have auras the heal nearby targets automatically, and the weakness/challenge of the spec would be healing the range characters while still attacking. This would mostly be done with beacons and some new mechanics. Maybe the healing cone spell could shoot backwards when pressed.
Maybe Holy Prism could be a placeable object that relays your AoE heals. The could even be a talent row that lets you augment it to follow players, or focus the bonus heals into one player at a time, or damage mobs as well.
Disc priest is my favorite to level right now. I really want an Uther playstyle, but the mechanics have to allow a melee-centric playstyle.