The only real niche left is reactive heals. That is, it goes out before the damage and heals it off after it happens. But, outside of flavor, this is pretty much how SCH operates with pre-Shields (and, well, it wouldn't technically increase HP like shields do). Still, it's what I'd personally like to see for their primary healing gimmick.
There's also indirect healing, generally done via damage that converts into healing(you DPS the boss, you heal people around you/people with a buff you applied first/people in ranged of the boss etc). Examples would be the excelent Blood Mage and Disciple classes in Vanguard which are probably the best healers I've played in a mmo, to an extent Disciple of Kain/Warrior Priest in Warhammer and Chloromancer in Rift(very close to bloodmage but not quite as good).
Astrologian doesn't sound like it'd use that type of healing though, thematically it tends to be melee classes or classes that have an affinity for damage stuff(like Blood Mage, Blood doesn't sound like your typical healing attribute).
Reactive healing sounds more likely and is an archetype also often used, though it's generally complimented by more direct heals or absorbs to make full use of the mechanic. WoW's shaman for example with Earth Shield had reactive healing, but it was a fairly small mechanic. Priest has that bouncing heal thing that was reactive but was more of an aoe mechanic(unless double tank stacked on top of each other sharing damage, then it'd bounce back and forth efficiently). EQ2 had some healer that was mostly wards and reactive healing, I wanna say Defiler, but I don't remember that much as I played DPS during EQ2.
Wards are also a mechanic, but generally extremely hard to balance. Think Mana Wall/Perfect Dodge "heals", probably with multiple uses damage reduction wards(absorb 20% of the next 5 hits for example). I don't think they'd build an entire class around this as a core mechanic due to abuse from stacking the healer class or the flavor not being powerful enough and leading to bland design.
From the design of other healers, I'd say the class will have mainline heals of the same type, or maybe leaving a reactive heal on the target(similar to how scholar heals leave an absorb shield) and the cards will serve as situational tools to differentiate the class, for example providing Apocatastasis effect(I think we'll see more of this overlap beween class like the Ninja and Warrior debuff, since more classes are being added but the raid size isn't being increased, kinda like how wow did it), potentially Mana Wall/Perfect Dodge with a debuff to prevent stacking, maybe a reraise kind of mechanic and so on. Sounds a bit like Aetherflow charges though, so might have a twist on how they recharge/how they're used.