>>532170756
>character already developed across the game
>already has a Social Link
>already has a boss fight
>already has dates/romance
Adding in a bonus extra, for fun only, playable Marie (only if you save her and only if you fight Izanami) for the final dungeon and true ending would be perfectly fine.

Ideally she could be added in elsewhere and be playable for a substantial period, but the ending just makes the most sense to me if that were to happen. I don't think it actually will though. They're already gonna be integrating her into other parts of the game