I'm assuming the potency scales off of healer potency and not the character receiving the potency. The GCD buffs would also need potential to scale as many DoTs as a SCH has in potency. Assuming no one missed GCDs, this can fix the issue of balance. Assuming SCH gets their DoTs for debuff, AST does the opposite - good for job identity. Time Dilation being 10 second cooldown is also fair and can be used on defensive utility.
Making a GCD star and having it become a "theoretical star" which affects your card's divination effects sounds like a great idea. It's like combusting an enemy with a "theoretical sun". Haha! I love the idea.
Though, I'm having a bit of a tough time understanding what how you would want it dynamically applied. From what I understand, ASTs won't be spamming it so... the AST won't be spamming malefic, but mixing Malefic with this GCD buff? Or is there multiple GCD buffs? Or is buffing going to be a complex thing? Sounds tricky to implement imo.
The are a couple of unknowns when giving AST back it's Time Mage identity though - exactly how much of it is it getting back, and what is the job development direction going to take from that point forward? ASTs would always want this buff up. Naturally, upkeeping a defensive buff on the tank is key so it would be best to Time Dilate onto the tank. However, both SCH and WHM have clear solutions to outgoing raidwide damage. ASTs as a Time Mage does not. How much burst healing will ASTs trade for Time extension abilities (looking at Time Extended AoE Stun Celestial Opposition vs AoE Heal Celestial Opposition)? I'm not saying it would be a bad thing, but ShB made it clear that certain fights require a lot of AoE burst healing. A Time Mage with only regen ticks cannot achieve that level unless they took a more mitigative stance with defense buffs being applied to everyone nearby. It's a difference in healing potential & healing style for sure and sounds a bit more hectic for balance and future content depending on how the job proceed forwards. Just food for thought.
Alternating Crowns without consuming a card sounds good provided it has 2 charges in case you can't Lord at the moment and Lord/Lady shares the cd in order to always get the CD up as soon as possible. That way you can depend on the oGCD heal without planning the next x CDs in advance.
A healer's responsibility is to heal unavoidable damage, not adjust to people who don't do mechanics. Healers manage mana and time. RDMs have quick raise. I'd adjust but there's a limit to how much I can adjust because I have to take into consideration of the party's survival rate too.