An idea for modeling the Phoenix Force:
Have it be a power set with psionics and fire control, etc. But add the following SFX:
SFX: Power of the Phoenix: You may add a second effect die of any size while using a power from the Phoenix power set, with no PP cost. However, you must add the same size die to your Emotional or Mental stress (player's choice). If you are emotionally or mentally stressed out in a scene in which you have used this SFX, activate the Consumed by the Phoenix Limit.
Limit: Consumed by the Phoenix: The Watcher controls your character for the remainder of this scene. You regain control during the next transition. You have d6 trauma; each time this limit is activated, step up the trauma. This trauma cannot be recovered. If it goes beyond d12, the Phoenix asserts permanent control.
Or something like that. This way the Phoenix can easily step up power to kick major ass, but also be in danger of being taken over by the power. And a canny player can play chicken with extra effect die and their stress, trying to use the power without losing control. And each time the player loses control, the Phoenix gets a little closer to taking permanent control. And the ongoing stress/trauma of using the Phoenix force becomes a bonus for enemies trying to goad her into action.
I've been wrangling with how to abstract it, and this was one way to approach it. Thoughts?