Tangent, but the more I think about what interesting support and healing features may be possible, the more I come to hate the idea of a cure-all/undo-mechanic Esuna, especially where spammable (no CD or low-CD and multicharge, and without limiting longterm resource costs). Hear me out.

Imagine we actually have some specific buffs. We'll start small. Take a CNJ, for instance, with its baseline Earth, Wind, and Water. Let's say we can use the Earth to Fortify allies, increasing physical defense, Wind to increase movement speed (not scaling with the target's existing movement speed), and our Water actions remove additional stored periodic damage or healing absorb based on its healing done.

Now, let's say there are specific afflictions, such as Ice/Cold/Frost/Freezing damage (whatever you want to call it), which as a Trauma (an affliction that lasts, at proportionately and increasingly reduced effect, until healed for the damage dealt, with its maximum duration --if any-- counting over time as proportionate healing done) will reduce its victim's Speed (movement, global recast, auto-attack, and casting).

You can therefore choose to heal that amount as quickly as possible to free up all those effects, or simply counter the debuff with a buff of your own to speed them out of would-be danger (especially since Wind's flat bonus to movement speed would be that much more noticeable when slowed). Or, as a WHM, perhaps you could counter the effect with Bravery, causing all effects against them to deal a flat portion less effect (a 50% or 30% debuff being reduced to 20% and 0% respectively). Etc., etc.

Esuna, unfortunately, just removes all that decision making. Instead of choosing between dealing with a specific portion of a debuff, trimming all just enough for now, or dealing with all of them simultaneously but less quickly, you simply have an "Undo mechanic" button. The only reason it'd ever have competition is the overpoweredness of AoE heals that, frankly, should be far more limited from the start.

Neither AoEs nor Esuna should be so strong. AoEs should be changed more towards weaker smart AoEs, as to not so anti-synergize with spot-heals and personal defensives, and/or be more limited by MP, gauge, and/or CDs. Immediate 100% cleanses like Esuna, likewise, should be more limited by MP, gauge, and/or CDs.