So many good ideas to include Phoenix into SMNs arsenals! 1 idea that I have is something similar that you do for Fenrir(or had to do the 1st time). Collect the whispers from all the avatars(this time including fenrir and diabolos as well), but that's not all. Maybe having to retrieve several explicit items(one of which would be a phoenix feather, and other items that symbolize fire/life maybe 5 different items) and trade them to a new NPC with the whispers in your inventory. The NPC makes a new item that's meant to be a conduit. You then go to a secluded location(as with all the avatars) and you trade the new item to the spot. It then uses up all your whispers and there's a CS where the avatars are summoned and transfer power to the conduit in order call forth the power of phoenix. Could even get selt'heus involved and if you do, the energy created could possibly be sufficient in calling forth more power from Phoenix(i.e. awakening more of her power) that is released from Selt'heus in the form of Phoenix(albeit not her full power most likely, but still strong enough to be a prime avatar in this state. She then shows her gratitude and requests a fight to see if you are worthy of controlling her power. Her fight shouldn't be anything less than difficult. Have some "prime avatar" gimmicks for her like, she Auto-reraises randomly(but a minimum of 2 times) like Aerns but, anyone within 25~30 of her take decent~moderate fire/light damage. I like many of the abilities that have been posted for her, but I would think her 2hour should be a little more powerful. I think it should be like RR4ga(raise with no weakness and fully healed; with a few buffs like haste, atk/acc/matk/macc/whatever else would be proper) however, using her specific 2hour ability should wipe Astral Flow status.
I know these may bee farfetched ideas, but we're trying to make this game better and better, not keeping it exactly where it is. Change can be good, and we need to be the ones to make it(and hopefully have these ideas reach SE's ears)
