I was thinking something fun: a romhack of Pokemon that replaces the mons with demons from SMT/Persona, but with a few extra functions:
1. You don't capture them. Instead, when you defeat a Stray Persona it has a random chance of the Persona wanting to join you, or to drop an item and will always drop Yen
2. Igor or Philemon replaces the Professor, the other one that wasn't used becomes the Champion at E4. Members of Elite 4 are Elizabeth, Theodore and Margaret. As for Gym Leaders, the cast of playable characters (with Labrys and Marie being extra challengers you could go and find in their special Dungeon).
3. Each "Town" and "Gym" would be replaced by a dungeon, the "Town" being the entrance to the dungeon and a sort of Hub, while the "Gym" would be the Dungeon itself.
4. As for the "Criminal Faction", I was thinking it would be the Shadows, with Izanami as their leaders. Their role in it is to "Ensure every piece stays in their place for the great ritual"
5. The rival would be Tohru Adachi, claiming he is in this "game" to make Izanami's ritual succeed, but in truth he would be siding with the heroes, visiting them to warn about the risks of breaking the pieces before the time is right. His attacking you is just his way of testing your worth as the "new hero".
6. The "Arcana Card" replaces badges, and functionally it acts as an unlocker for the Ultimate Persona of each Arcana.
7. If there are far too many MCs, then every Gym Leader Battle is a Team Battle with the characters of P3 and P4 of matching Arcana or role.
8. You would play as a character specifically made for this, of course. The player would always be mistaken for a Shadow because of their eyes... Well, it wouldn't be too far off, the player would be a Shadow in pursuit of his/her true self. End goal would be to meet their true self.
9. The Personas would evolve to the next form in their arcana, and every level would have a chance of a random skill being given.