I think whilst "support" as a role would be too late to add, I definitely think there can be more in the way of supporty stuff, rather than everything feel like a variation of pure DPS. But I come from a background of playing COR in FFXI where it felt like I had a variety of buffs I could interchange based on the fight and situation and it felt more meaningful to the mechanics of the game. Whereas here, we had some level of it, which has gradually been gutted out.
But if I look at what COR could offer me in terms of support back in FFXI, there are probably some things (but not all) that can translate here, but here's a list of buffs that exists in XI that I think would work/have worked here:
Save TP*
Regain*
Haste
Snapshot
Fast Cast
Peloton
Buff Duration Boost
Conserve MP
Regen
Boost Magic Attack
Defense
Double Attack**
Crit Rate
Refresh
Store TP*
Cure Potency
Magic Defence
Attack Boost
* - If instead of removing TP they took it the other way to make TP more meaningful
** - Although the mechanic only half exists, but "next attack hits twice" would be a cool adaptation of this buff.
That's a pool we can choose from that (aside from TP) would not require changes to how the game works, but of course changes to balance. I think having a support DPS job pulling from the above could be fairly adaptable. Because ultimately the rDPS is most important when it comes to balance and if the BRD/DNC/whatever jobs get support are limited to what they can apply at a given time I think a good rDPS can be balanced whilst still having all of this other stuff to buff.
Because it's stuff like haste, crit, double attack, attack, magic attack and so on that would be an rDPS boost and if they all feel meaningful, then great. But then we end up with extra abilities that are more situational or maybe better in progression groups where they struggle more, so your regen, cure potency, defense boost, conserve MP and so on would be beneficial. Though for the latter the healing in me says "but we heal too well already, this would increase our downtime", so I think for those kind of buffs healers needed to be weaker and that's something I've already asked for on the healer forums.
But there is a drawback here. The more useful the buffs are, the more "required" a buff-centric job is needed, so that DNC or BRD may be considered a requirement in a party make up rather than say, MCH, who is the selfish DPS of the three, unless we can get MCH balanced to BLM/SAM/SMN levels of DPS make it so MCH is competing with their space instead of BRD or DNC
The second drawback is that whilst all of the above would be cool from a support job fantasy perspective, borrowing from a title that made use of the role, once the most efficient rotation has been set up, being given a choice to manage buffs could become less meaningful and more a part of a rotation, and SE has streamlined how that would work in BRD by making a "buff" rotation in the form of songs and this would lessen how supporty it feels because you've not made any meaningful choice behind the support you've given. So I'm not really sure how they could tackle this. AST had the advantage of RNG, so they make sure of what they got and had to make a choice in how they used their card.
But I expect there is a way. However, the other problem will come in the form of people who enjoy BRD as a DPS job, rather than the support side. Whilst traditionally bards are support orientated, SE also made BRD a hybrid of Ranger and Bard, so those who favour a ranger-type job are also playing BRD. So they'll have to dissatisfy them to make it work for those who want more support orientated stuff.