Another challenge, this one for a Persona 4/Kingdom Hearts crossover or a Persona 5 one.
All of you know Xion, right? And her tragic end... And that will, somehow, return in Kingdom Hearts III. Well, there are some fanfics were Xion revives in another world like in Second Chance as a Pirate, of Coral The Leviathan and Tides, from Prince Arjuna. Those two take place in One Piece and High School DxD respectively.
Well, what if Xion is too deep in Sora's heart when she returns to it that makes contact with the collective unconscious and is transported to a world where people's deepest thoughts and desires materialize?
That's where the Persona side takes in: somehow, Xion appears in the TV world (P4) or Mementos (P5). Once there, she's discovered by the Investigation Team or the Phantom Thieves of Hearts, respectively. Oh, and she acquires a Persona, too.
About Xion's capabilities in the two sides: she can still use her Keyblade in the real world and the other, including spells and attacks; her Persona, preferably, must be of Water element (Xion's ice spells will be categorized as Ice in the system) and have spells and skills that she can't use with the Keyblade alone.
Now with the P4 plot: when consulting the Midnight Channel, Yu Narukami and his friends are spectators of some sort of corrupted emission, that Teddie thinks is because of some sort of anomaly in the TV World. Once there, they discover a strange dungeon with an even stranger girl called Xion, that thought she died.
Well, in that case, Xion's program and dungeon are based on her memories of her time in Organization XIII and her Shadow could be her repressed desire to live or something like that. It could occur after Kubo's or Naoto's dungeon.
After that, Xion could be attending Yasogami High as a first year (biologically speaking, she is 15) and I don't know, she could live in Amagi's Inn or some apartment.
Her Arcana, it's difficult in this case. Marie's Aeon so Xion's can't be that, unless she's in P5, therefore she could become the Aeon.
In Persona 5, it happens similarly but not the same: is by chance that the Phantom Thieves encounters her in a distortion in Mementos. Post-Kaneshido or Futaba.
Like in P4, she could be a 1st year at Shujin. And her thief costume is only a modification of her black coat (e.x.: blue wave motifs and a seashell mask). As for her ranged weapon, I would say a sniper rifle.
For a bonus, there could be an extra dungeon either in the TV World or the Metaverse modeled after the Castle of Oblivion that could serve the same as the Datascape: they dive in copies of Sora and Xion's memories and at the end, there could be a clue as for why she's there or her role in the next Keyblade War.