Thanks! The classes of Warhammer Online sound like they were fun!
Now, looking at the current situation, we have everything covered as far as healing goes. We have HoTs, single heals, AoE heals, damage mitigation, and buffs. Any new healer would be redundant on a lot of fronts. In addition, if the new healer is too good, players will be less inclined to play the current three. But, if the new healer wasn't good enough, then the question of why the new healer was added in the first place comes up.
Certainly, a big issue is that our boss fights don't allow a lot of variety in healers; but, as you pointed out, that may be a blessing. It's far easier to balance a few jobs than it is many.
I'm inclined to believe unless there is a complete overhaul of FFXIV's dungeons, and our existing healing jobs, there can be no additional healers added.