So they're actually experimenting with a support spec. Awesome news, and I think the follow-up to that should be changing an existing spec into a support spec.
I think Priest is the best candidate.
My topic points:
We already have 2 healing specs! There's no rule to how many healing specs a class should have, but 1 feels like enough to me.
We already have a history of having the most player-interactive abilities and single-target buffs, as in something other than providing a heal - Void Shift, Mind Control, Leap of Faith, Power Infusion come to mind.
Discipline always felt like a support spec already. As my main, I vouch that we've always had a playstyle unique among healers. We had our glory days of being able to single-handedly heal a tank and a raid at the same time during MoP and especially SoO, but since then I've felt our place has been as a back-up healer, or off-healer, or however you describe it, Diet healer? This IMO fits perfectly with the description of what a Support class can be.
What would MY ideal Discipline-as-a-support-spec look like?
Remove the healing component of atonement, but bake in the Lenience talent. Give atonement a component to the buff that provides resource gain depending on the class used on (Similar to Blood Elf racial) little bits of extra mana/rage/fury/focus/runic power every few seconds.
Give Power Infusion multiple charges, like 3 or 4, so that it could be used strategically to buff multiple players prior to a pull (Like a party-sized lust), or to keep the buff rolling on a single player with cooldown management.
Give Dominate Mind a longer pet duration
Give Fiend/Bender a longer pet duration, these two will provide Priests with the much discussed fantasy of having pets. Mind Controlled servants and a shadow minion would fit the fantasy of a Priest.
Remove healing component to PW: Radiance, used as a cooldown to give the entire raid atonement. Which with mentioned above will be the damage reduction % and resource gain only.
Give Mind Blast back the damage negate component it used to have
Give Pain Suppression a shorter cooldown and a longer duration. Keep Transform Pain and Protector of the Frail as talents
Move Archangel and Dark Archangel to the PvE tree, in place of Lenience and Evangelism. Lenience is baked into atonement. remove Evangelism altogether
Make Flash Heal expensive and painful to use, similar to Shadow Priest.
Keep Barrier as it is
Keep Rapture as it is
Keep Power Word: Shield as it is. Keeps atonement rolling
Keep Smite, Penance, SW:P as they are. PtW continues to spread by Penance.
So, on top of PW:S being technically an absorb, this leaves Disc Priest with the healing spells of Renew, Flash Heal, Penance, PW: Life and the healing from Nova, Halo and Star. Not enough to heal a group through hard content, but enough that a decent disc priest can heal lower content, can help out a struggling healer in the group, can do enough to hard focus on a tank if a healer drops to carry them through.
So why bring a Disc Priest? We are left with abilities that buff a tank's ability to tank, buff a healer's ability to heal and reduce incoming damage, and give DPS a powerful cooldown. We are also bringing a modest amount of backup healing and DPS.
The people who like healing as a priest have nothing to lose, the people who like Discipline should still like it as the playstyle remains intact albeit for a different purpose, and the people who like healing as a Disc and hate healing as a Holy get one step closer to seeing a therapist.
This is of course all dependent on Augmentation not being a flop, and people end up liking a support spec in their groups.