I know I don't have AST at 80 yet (working on it), but, seeing the frustration over what it is from my raid co-healer, and looking at what tools the other healers have, gave me some ideas on what could bring AST up to snuff.
1) AST doesn't really have a shield it can lay down. Yes, Collective Unconsciousness is supposed to be that shield, but the AST has to channel that. While I think it should get buffed (increased Regen potency and have the shield effect also be a timer buff that gets refreshed while under the dome), I think a secondary effect should be added to Earthly Star. It's already an ability AST can place, so I think it getting a secondary Regen/Shield effect while standing in the stars (depending on stance) would help AST (as well as encourage people to stand in the thing).
2) Celestial Opposition should grant the opposite buff, not same. I don't know what it's called in Japanese, but I really think this should grant the opposite effect of your current stance, considering it's called "Opposition". If it could also grant MP like Assize, that would be great. The idea behind the ability (at least originally) is that you're turning back/freezing the clock.
3) Celestial Intersection should be a buff like Emergency Tactics. It temporarily turns your next Aspected cast into its other cast. It could also affect Celestial Opposition if they would rather keep the current effect. It might need a longer cooldown to compensate, and I worry that this idea will make Intersection somewhat superfluous.
4) Divination feels weak. For it being the main thing AST works toward, it really doesn't seem to do much. While I don't know just straight buffing the amount or the duration is the right call, I do agree with what someone said about bringing back the old Time Dilation effect for cards into Divination. However, this could lead to issues with Sleeve Draw and Divinations' cooldowns not aligning cleanly.
5) Synastry could be used to share buffs. I think making it act like a temporary Dance Partner would make Synastry more impactful. So, if you link someone and play a card on a different person than the one linked, the linked target also receives the card.
6) Redraw's charge time. 30s seems a bit too long for it, especially when we draw one card every 30s now. I was thinking maybe 10s, but then they might think it would be too easy to get the right seals for Divination.