- Keep their Time Mage aspect relevant by making them the most mobile healer. Putting the regular Benefic on an instant or even a 1 sec cast makes it a great tool for emergency intervention and for weaving in a ton of oGCDs. Benefic would probably be their only single target direct healing tool on the GCD, perhaps with a potency reduction to 350. I'll elaborate more on that on point two. Even the cast time on Helios could be reduced in the same way to make it a quick (though not instant) AoE healing tool. I don't think removing Helios would be a good idea, as it would seriously cripple their AoE healing capacity, and it's a basic enough spell for them to keep. Might have to tone the potency down to 240-250 though.
- Give them the strongest single-target regens in the game, but keep their direct healing options weaker. More or less, make Asp. Benefic's 300-500 total potency stronger than Regen, with also a slightly stronger heal per tick, at the cost of them losing Benefic II. This cements AST as a support and utility healer which is assisting more than directly healing, and further identifies the job's utility. I also think it would be interesting if Asp. Benefic was actually a CD with a 15 sec recast instead of a spell, and if it had a proc interacting with either itself or Benefic that reset the timer, kinda like how Low Blow works on DRK during HW, and also keeping with the RNG aspect of the job without being awfully punishing (since AspB. is in a shorter CD than its full duration).
- Asp. Helios could also be an oGCD, giving it a decent-powered HoT similar to a roused Whispering Dawn, probably with the same duration (18-21s) and on the same CD as WD or even a slightly longer one, since Celestial Opposition should still be a thing. Additionally, there could be CDs related to the use of the regens that could transform 50% of the total potency into a shield similar to Nocturnal Sect's effect. Either this, or a CD that would accelerate the remaining potency of every HoT cast by the Astro into an instant burst heal. Could also be toned down to 50% of the total potency cause it could border on hilariously powerful healing, and should probably be on a quite lengthy CD. Perhaps if the total duration was of 18s with a potency of 150 per tick (Rouse+WD is 140) it would be good for both the regen and the burst: the total potency would be 900, and if cut to 50% it would be a 450 instant heal on a lengthier CD than Indomitability. By using Celestial Opposition, this increases dramatically, but has used up 3 different CDs to achieve a great single burst healing.
- Both perviously mentioned CDs (the shield and the burst) could be a thing and probably shouldn't be mutually exclusive. Then again, they could also have their own Nocturnal Helios kind of CD on its own, without needing to sacrifice the regens for it to work. The shield should still probably not stack with Galvanize, and if Stoneskin II was still a thing (and 90sec CD useable in battle, as it should be, imo >_>) it could stack with it to give AST/WHM a decent mitigation tool that couldn't be affected by healing potency+ buffs.
- Essential Dignity should probably still be a thing, as well as Collective Unconscious. One offers a good powerful heal for emergency single target intervention, and the other offers good mitigation and an extra powerful regen for when Asp.Helios is on CD if needed. Perhaps CU's cd could be increased but it's debatable.
- By keeping with the theme, I think Synastry would be a more interesting ability if it instead connected the AST to the target ally, and in turn the targeted ally shared AST's personal CDs and durations used as long as the Synastry connection is up. If used again on a different target, the two targets are instead connected, but the original target doesn't lose the CDs previously shared with AST until they (or Synastry) expire. Similarly, the new Synastry target would benefit from the same buffs and effects used on the original target. For healing purposes, you could connect the two tanks together, and only apply a regen (receiving the buff but not the initial direct heal) or a Bole on one to extend the effect to both. For offensive purposes: imagine giving an enhanced Balance to your BLM and then extending it to your SAM.
- A new creative ability that makes a return from some past games in the series: Float. A personal CD which can be shared by Synastry that allows the AST to ignore ground effects (like fire puddles, frostbite, etc) for a short duration (10-15 secs tops). It could also add a +25% movement speed buff to keep it universally relevant.
- Allowing Time Dilation to be used on oneself, which helps with the concept of Synastry, and would also allow AST to share the effects of Lucid Dreaming, Largesse, etc, etc. Might have to tone it down to 10 extra seconds per use if this is the case, though.
- And finally, the crowning skill and the main reason why the shield probably shouldn't stack with Galvanize: A personal buff, shareable by Synastry, that increased Critical Hit rate by 50-100% for 3-5 seconds tops, and on a pretty lengthy CD (5mins or more). Yes, I know I'm gonna get flack for this one, but hey, we need synergy with both WHM and SCH :P. This could give a potential guaranteed critlo every time this CD came up, or at least two GCDs of Cure III guaranteed crits. Obviously, WHM would benefit less from this than SCH, and I'm aware it's probably an incredibly problematic skill, but thought it could be an interesting addition that would make AST+SCH comps more desireable (consider that these changes to AST would make it a very difficult healer to play through progression, as, just like SCH, all the CD usage could be overwhelming and the tools aren't nearly as forgiving as WHM's).
- Earthly Star could probably stay, though I'd personally extend the CD a little bit to keep AST's healing power from creeping into WHM's, as they'd already have pretty strong burst healing options.