Balance Druids in WoW worked similar for a while, though between two aspects of DPS instead of DPS/Healing. But yeah this'd be sensible. It still requires healing expectations across all content to massively increase while also greatly curbing the support/ability pool of all healers (and even non-healers) so that GCD healing is far more required as an activity, but this in turn would then allow this "Heal a lot with GCDs -> Unlock a supernuke // Nuke a lot with GCDs -> Unlock a superheal"-setup.

On that note, I am easily accepting of 3-5 extra damage buttons on healers but only if 3-5 or more healing/support buttons get axed in turn to not further cramp the already laughably overloaded hotbars all jobs have in FFXIV. This would be easy in so far that healers have an absolutely bonkers amount of oGCDs, which in a lot of ways all do the same thing.

Example:
* On Scholar, the only truly outstanding oGCD is Expedient. Eat-Fairy nearly qualifies but since the whole mechanic with the charges vs fairy vs gauge is super-pointless and undercooked right now, it serves no purpose. All other oGCDs are just fancy variants of heal more or heal faster. These could trivially be removed in turn for improving the main healing GCDs if even wanted, and make space for more damage skills. Not nearly all need to be removed of course, but everything would qualify.
* On Astrologian, the only interesting oGCDs to keep are either Draws and Plays or the "prediction" skills Star / Horoscope / Macrocosmos. Which set depends on your outlook on what defines Astro conceptually of these two things. But again, of course both can be kept (and everything else like Sect / tankbuff / CO removed), but in extreme cases all but 4-5 buttons can be folded into base skills.

It's not better on White Mage or Sage either. Sage already does this smarter anyways, since it has just one truly important oGCD button, Eukrasia, and can dual-use all of its other buttons. All of these healers could easily lose half a dozen oGCDs and make room for 3-4 more damage skills, ideally GCDs, and turn another 1-2 oGCDs into healing GCDs to more evenly balance the scales, the implement such a back-and-forth system, though conceptually maybe only White Mage should have this particular one.