Well, that, and Prayer of Healing. 
For reference:
While Heal does have Lightweaver to bolster it, having the ability to give it extra oomph could be nice, not to mention getting closer to that feeling of Greater Heal at a longer cast. Very unlikely we’d get a % based heal in PvE since it’s very limited on other classes.
Maybe fully empowered it (and PoH) grants Renew on the target.
Edit: this would bake in Everlasting Light so that could be something else.
I could see PoH having this treatment but not heal tbh
Empowered aoe healing spells could potentially solve Holy Priest’s lackluster aoe healing performance. That would be really cool!
This would make me stop playing completely.
Hate how empowered spells feel to use and holy already has enough issues with being stood still to cast for too long.
I can’t say I’d be in love with the idea of shackling Holy with empowered spells.
We’re already not a very mobile class. I’d hate for even more of our power to be reliant on standing in one place when that’s not what the game’s designed for anymore. Especially since, unlike Evokers, we don’t have the tools to let us cheat them out at full power in a pinch.
Plus, I just find the mechanic a little janky, unfun, and unreliable in general.
The game moves too fast for me to be dealing with that. As it is I already don’t cast Heal because it’s too slow. If a heal isn’t going off in a half second I’m at serious risk of it being a wasted cast. Being able to slow down and cast a powered up Heal just isn’t something that’s going to work with me, not with the pace this game now plays at.
I do miss the days of Greater Heal and mana management.
You’d never cast heal without a lightweaver buff anyways.
I think that’s part of why I was thinking it (after watching a friend play Evoker). I don’t like Lightweaver, and I like long, powerful casts (managing to land Greater Heals was fun in PvP). They feel good when there’s time to do them, and set us apart from practically every other healer which is more mobile—I’d rather not be mobile, but that should come with trade-offs… and Phantom Reach as a capstone on the far end of the tree shouldn’t be it.
… Back to my point there… I don’t like Lightweaver. I’d honestly rather use Lightwell in M+.
Slower, powerful casts could also go well with Ray of Hope, and I’m going to keep putting PvE Ray of Hope out into the universe.
I suppose if it were only one spell, though… Prayer of Healing would probably be preferable, giving it the potential to be strong, well-positioned filler, with Circle of Healing and Rhapsody (which I also don’t care for, just make Holy Nova big and hit hard and have a CD) Holy Nova for emergency group healing during movement.