You think current rdm vercure/magick barrier would be enough as a damage spec, would outright replace healer? then that's a problem with current design of healers. That's what I would generally give it, possible to add more but you'd need to be careful for reasons you have stated, you don't want a RDM in a DPS build to replace or be good enough that it can act as a healer
The healing Spec would
1. add healing trait abilities and healing spells, so basically what healers currently have.
2. add a general healing buff to
The Damage spec would
1. Add a strong magical damage potency (To generally make it on par with other dps)
2. Some damaging spells, mainly strong burst options.
Imagine a sort of skill tree where you can go into two directions one being a (healer) orientated abilities this would include healing potency biists, the other would be (DPS) orientated abilities, this would Include damaging potency passives
a Red mage using the "Black Shift" would not have most healing spells (likely just vercure/magick barrier) while a white shift RDM, would have generally weak potency spells and lack some of the real bursty ones (to the point where it's dps would be the same as your average healer), White shift RDM.
I'm mainly using Red Mage as a baseline as it already has "healing" abilities it's easier to imagine RDM as a healer, but this could apply to tanks giving up defensives ect. PLD would trade in it's Hp, stance ect. But retain some of it's rotational and baseline abilities and be more adapt at healing.
Like I said this would be pretty difficult and unrealistic to balance. But yeah I do think hypothetically it could work? It just would never be implemented into current ff14, the current design is super rigid and wouldn't allow for choice expression and difference specs, having to balance 2 specs per job (with optional stuff inside those specs) would be very tremendous to balance, it wouldn't likely be "good" for the game unless they manage to actually balance it well, which again is a high task, I just like the idea of Jobs being more flexible and having different builds, (Crazy I would enjoy something that not everyone else would).
EDIT: Strictly for arguments sake you couldn't use any abilities or passives of "White Shift" while you are using the "Black shift" action (vise versa), think of it like selectable role actions, you choose white shift and have access to a lot of Red mage healing traits (you couldn't change mid duty)