My suspicion is that Devotion Aura and Ancestral Vigor will carve out their own raid slots, so the remainder are left for Holy Priest vs. Druid vs. Monk. I'm not sure that's a comparison where Holy Priests end up looking all that strong:
Innervate vs. Hymn of Hope. With 4 healers, Innervate is half the power (1 healer, twice as often) but it allows you to pick the specific healers that benefit most at the time they benefit most. So it's a bit of a wash.
Tranquility vs. Divine Hymn. With the artifact weapon, Tranquility is significantly better than Divine Hymn. Not only does it heal for a lot more, but it can be channeled while moving.
Ironbark vs. Guardian Spirit. These aren't directly comparable, but the rapidly reusable Ironback tends to be favored by many raiders.
Given that Druids are also the most mobile and durable healer, I'm expecting plenty of "whither Holy?" discussions as Legion raiding rolls onto live.