One idea I've see thrown around in the past is to make Fey Union's behavior change with the fairy. This idea is usually accompanied by a rework to Selene to change the fairy decision from healing/support to ST/AoE. So Selene gets current Fey Union along with a rework to have abilities more centered around single target healing/buffs, while Eos get the new AoE version. ...though no one seems to think to specify if their idea of the AoE should center on Eos or the target.