I can definitly see the potential in something like Plan A/B/C. Imagine it like stances, start off with no plan, no bonuses. Can only enter them in sequence and can go through them fast, can not go back to previous plan, can only drop a plan after 5s. Then Plan A (Attack), increased damage, healing down. Plan B (Bolster) increased effectivness of shields and fairies. Plan C (Crisis), increased healing on all abilities, Liliy stuck on Sic, damage done greatly reduced while plan is in effect. Also love if both some healing and damage spells get secondary effects in the different Plans.

And if we are to be with the pet-system like Summoner I would love to see it more involved, like you said back to have different abilities on Eos and Selene. Would be fitting to have them with unique abilties on individual cooldowns and in return increase summoning time and mp cost. Then be able to either instantly swap them on a longer cooldown or AF cost.

Have no worries about potencies, can be tweaked after the playstyle is nailed down.