I get the sneaking suspicion that AST's won't have strong offensive potential compared to the other two healer's in the game. I would like their stances will be a heavy healing stance and a light healing support based stance.

What I'm thinking is the healing stance will be provide a boost to their healing potential, something like a permanent Divine Seal (but have lighter potency heals to make up for that).

In contrast, their light healing support stance will have their heals be small AoE (10y?) HoTs with a healing penalty (something in vein of how Cleric's Stance reduce potency) that while the HoTs are applied, the respective party members get a small buff to their stats.

This way AST can provide a heavy healing role or a supportive healing role and be matched well with either WHM or SCH at the same time. Being able to stance dance effectively would also be extremely rewarding as well as you're providing the best of both worlds to your party.