So, reading that other thread someone suggested playing with polarity and having some powers have negative charge, some positive, and interactions between the two. Had me thinking, could add some buffs that when combined would be rather OP, but with a pole system, not letting them be combined. For example, have one buff give a ton of def, and another a ton of resistance, but both buffs have a negative charge which would be replant to each other. So someone can have one positive, one negative and as many neutral buffs as they like. That or have 1 power have 2 different effects depending on your own polarity when casting, but only 1 version of the power could be in effect at once (although depending on how many of the powers use that mechanic, it could lead to having double the powers in a single set).
And perhaps add a mechanic where two teammates with separate like charges interact, creating a pulse that debuffs nearby enemies.
As for the chain targeted AoE heal attack, maybe have it have a middling-weak base heal and a second heal that increases in power proportional to amount of end drained, so the heal would be strongest against foes at the start of the fight or after allowing the enemy time to recover, or you can keep hitting with a weak heal to help keep them sapped.