The concept of a "hybrid" has always been a tricky one to tackle in MMOs, but it's certainly doable.
The idea behind a hybrid is that the added utility and versatility the hybrid will bring to a group makes up for their lack of strength in any one specialization.
In the case of a hybrid healer, we must first consider what the role of a healer during a boss battle truly is. The obvious answer is "keep people alive", sure, but that's not ENTIRELY accurate...
The REAL answer is "keep people alive until the end of the encounter", and there are multiple ways one can go about doing that.
A WHM focuses on healing damage that has been taken. A SCH focuses more on preventing the damage from being taken initially through mitigation.
By design, an AST was intended to focus on ending the encounter more quickly, thus negating the need for as much healing and mitigation. Just look at the cards: the only card that doesn't directly work toward ending the counter faster in some way is Bole (which mitigates damage).
Everything else either causes more damage (balance, arrow), makes cooldowns end more quickly (spear, which leads to more damage), or encourages players to spam abilities that would normally drain their resources too quickly (ewer, spire).
Yes, some of these can aid in healing as well, but all but one has a component that in some way aids in doing more damage. This was DESIGNED to make encounters end sooner in exchange for not having the emergency buttons and heal-boosters of the other healers.
And as I said earlier, this CAN work, but the AST has to legitimately deliver on ending the encounter more quickly, and right now, we have enough data that has proven that this just isn't the case. In fact, SCH DPS outperforms AST DPS, and the reliability of the Selene buff is just way better than the current randomness of the cards.
The AST was also designed with a risk vs. reward playstyle (which I absolutely LOVE) but this will only work in the long run if the risk is truly worth the reward. Otherwise, ASTs will be relegated to content that's on farm status, and I really don't want to see that happen to the class.
So here are a few suggestions on making AST end encounters more quickly, and no, I'm not suggesting ALL of these be implemented together, but maybe one or two here and there:
1. Significanly Increase AST's Natural DPS
Just make all of the AST-exclusive spells do more damage outright, and if they wanted to REALLY give the AST a leg up as a DPSing healer, make Cleric Stance unnecessary and calculate its spell damage off of the mind stat instead of int.
AST has enough buttons to push and would benefit from not needing to swap in and out (plus, you wouldn't have to worry about lag spikes causing CS to not deactivate when pressed).
2. Double or Triple the Potency of the Card Buffs
AST's have to average out to doing significantly more damage than other healers to be worth bringing, and that factors in the RNG nature of the cards. Remember, whatever additional benefit the card buffs can bring to an encounter is effectively divided by 6 as you have a 1 in 6 chance of drawing any given card.
Plus, pretty much all of the cards are limp noodles right now. 20% less T/MP usage when the AST happens to draw it is a joke. 20% cooldown reduction would be decent if it worked on abilities that were already on cooldown. A situational 10% damage reduction only serves to offset the lower mana efficiency of the AST. While 10% damage and 10% speed are nice, they're not nice ENOUGH.
Again, divide all of these by 6 and you have the actual benefit we're looking at.
Even firing off all of the buff duration extenders we have onto the top parsing DPS just doesn't make us worth it.
3. Make AST able to move while casting EVERYTHING
Many of AST's tools are already geared toward mobility (with some notably stupid exceptions like CU, which should also be mobile while channeling). AST could fill a role as a mobile healer, making them shine on fights with tons of movement/evasion.
4. Make the Cards Heal/Shield as well
A simple way to adjust for healing potency would be to add healing to the cards. For example, a card could heal for 1% of the target's HP (or a 50 potency regen) every second for the duration of the buff. This would also indirectly make ewer/spire more valuable for their 20 second duration as it would provide a longer heal.
Cards could even heal differently depending upon stance, maybe giving the the target a shield when in Noc stance.
This would also make the cards more versatile, as no card would ever be completely useless (like when you buff someone and a second later they get trapped in one of Titan's gaols) and the odds are that either the heal or the buff would be put to good use.
5. Reduce the Cooldowns on ALL Card-related abilities
I think it would be entirely reasonable for an AST to ALWAYS have a card buff running on SOMEONE, so I'd suggest:
Draw: 15-20 seconds and the cooldown starts the moment you draw the card instead of when the Drawn card falls off (I hate having to manually right click a card off because I'm not about to ruin my RR with a goddamn spear...), either that or, if you let a card expire after drawing it, you can immediately draw again.
Spread: 30 second cooldown and the cooldown begins once the card is held.
Shuffle: 30 second cooldown and it cannot draw the same card again.
6. Make Royal Road Not Suck
Seriously, there's only one situation in which you actually get more benefit for RRing and waiting another 30 seconds (expanded RR).
Ewer and Spire: Triple duration on the next card. If I'm going to wait another 30 seconds before buffing someone, it better be WORTH the wait.
Arrow and Spear: Increase the radius of the buff to 30y because good luck trying to corral everyone in the middle of a raid encounter.
Bole and Balance: 250% potency on the next card. Again, time is money, and just an extra 50% to wait another 30 seconds is a joke since you've just spent 200% card potency to get 150% back in return.
7. Make the Cards Less Situational by Giving them a 1 minute duration and a number of charges (after you've cast them)
Instead of throwing a TP reducer on a party member and hoping that they needed it for the next 20 seconds (when you have no idea if they actually will due to the randomness of boss mechanics), how about you throw it on a party member and the next 10 abilities they use will have their TP reduced by 20%?
Bole: 10% less damage taken from the next 10 hits
Spear: 20% faster cooldown on the next 3 CDs used.
Arrow: 10% faster cast time/GCD recovery time on the next 10 abilties/spells used.
Balance: 10% more damage done on the next 10 attacks (doesn't include auto attacks or DoTs already ticking)
This would ensure that buffs don't get so easily wasted as the player can use the buffs when they have a chance.
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In conclusion, I love the AST's playstyle, but the RNG needs to have enough rewards to offset the risks and that's just not the case right now. I want to see the AST be the luck-based, micro-managing, "run n' gun" hybrid that it could be, and TBH, it won't take that many tweaks before it's there.