I think the AST will have 2 stances, a heal/support and an attack/support.
If SE maintain the 2 classes crossable, AST will get the skills from BLM and SCH. In the ATK/SUP stance they will have their version of stoneskin and 3 cards to use their mudras (!!!). In the heal/SUP stance they will have a protect-like spell and 3 more cards to use more mudras, while sharing basic heals and attack spells with ATK/SUP. If they really work this way, AST will mostly have the biggest set of skills from the game...