4th Range DPS idea after endwalker

[Archwizard]

What time magic is there left to AST at this point other than lightspeed?

A solid way to answer this question is to look at the most common Time Magic spells:

Gravity and Regen are, in some way, available to Astrologian. Okay yes, "Aspected Benefic" isn't the same name but WHM has Regen anyway, just take what overlap you can get.
On a similar note, Meteor is already represented in game... through the BLM limit break. Arguably though, AST's upcoming "Macrocosmos" skill fits raining down Comets (in a healer-appropriate manner).

Haste has already been noted by Yoshi-P to be a spell we will never see available in-game, because he believes it would be wildly overpowered. (In all likelihood though, it would be considered the worst utility in the game since it would reduce your target's oGCD windows without their control.)
However! Besides Lightspeed, Astrologian's ability to buff allies' damage directly with its cards/Divination covers much of the same niche as Haste and the odd buffs it had in other entries. The upcoming "Astrodyne" skill will do much the same for AST itself.

Likewise, Reflect would also be overpowered, which is why we only see it contained to zones like Bozja.

Teleport and Doom... actually have precedent, thanks to the Trust version of Urianger having a copy of BLM's Aetherial Manipulation and his own execution-variation of Death. AST doesn't have them personally but, pin in that thought, maybe later if you can make an argument why it would need them (admittedly hard to do).

Most CC spells like Slow, Stop, Old, Disable, Banish and Mute would be kinda pointless to implement. Depending on what enemies they can affect, they would exclusively be either "completely overpowered, you would never take anything but an AST anywhere" or "completely useless, who even bothered programming these" with no in-between, which makes them more reasonable as BLU spells (where the point of the class is to pick the skills most useful for the situation, learning enemies' status resistances, etc).
We can also toss environmental buffs like Float and Invisible/Vanish into this pool too, given that instances would have to be designed with Float in mind, and a number of bosses already ignore NIN's Hide.

So all we really have left in the Time Magic list is Extend, which Celestial Opposition USED to be able to cover... but was removed because why waste precious hotbar space on such a button when the devs could just fold an extra 3-4 sec duration into AST's effects.

And, unique to Astrologian given their lack of precedent amongst Time Mages, are also effects like Celestial Stasis reversing time to resurrect and heal fallen allies, flavor of "predicting the future" attached to their card system, and a number of appropriate spells revolving around astral bodies such as Malefic and Combust, or delayed effects like Earthly Star and the new "Exaltation" and "Macrocosmos" skills.

You have to remember, "Time Magic" would be more accurately called "Spacetime Magic".

Really the question is, what would a Time Mage be able to salvage if implemented as a DPS job? Despite what you might think being a natural union, there's not much precedent of DoT effects amongst Time Mages, much less DoT manipulation. All you got are Bleed and Extend, and those aren't even available in the same games.
I mean, you could argue for a DoT caster and even name it Time Mage, but it would actually be scrounging for effects it could use because virtually none would fall under known Time Magic spells.