Honestly, the "buffs" would likely be oGCD short duration things with CDs. Think stuff like WHM's Divine Benison or RDM's Magick Barrier. That's what modern buff/debuff gameplay is in FFXIV. In theory, they could have some GCDs (like SGE's Pneuma that is a 2 min CD spell, but is on the GCD instead of an oGCD weave)

There are honestly lots of ways to do it, though I suspect the Job would mainly be a DoT based Caster with party mitigations/buffs/heals (Regen) as oGCDs of the 30/60/90/120 sec CDs. They could be GCDs, but that has more issues with the current gameplay unless they refund damage somehow (like WHM Lilies), which is possible I suppose.