Part of what makes this game function so well is it's adherence to the holy trinity, separating players into DPSs, tanks, and healers, but at a certain point that gets really boring. It sacrifices horizontal progression for consistent and well defined combat roles. I'm perfectly fine with keeping people to one of these three roles, but I think that a lot can be gained from giving people secondary roles, including but not necessarily limited to these three.
First off, this can lead to people neglecting their main roles, even in times of need, so I think that these roles should be dependent on performance in your main one. For example, if a healer can excel at healing their party, a stacking buff might be accumulated via healing to be used for their secondary role's actions, and if they want to run solo, they might be able to summon an aggroing dummy for them to heal to accumulate said buff and to use as a weak, pseudo tank.