An idea came to my mind after finishing Persona 5 and reading some Persona x Date a live crossover.

Unlike P3 & 4, P5 plot made it really fit into DaL story. With the Navigation app, Joker can go into the Spirits' Palace to change the heart. Just like how Shidou helped the Spirits by wooing them. And by defeating their Shadow, Joker can obtain their mask which gained him their power, like Shidou's ability. And that was my idea.

Basically, it's Joker going into DaL world, saving Spirits by changing their heart.

You can made it Joker went into DaL world or Joker went back home and discovered the Spirit. Personally, I preferred the former. It was more logical that way. You can have Joker solo or the whole Phantom Thieves. About the Palace, it would be full of Futaba-like Palace. Spirits were not evil after all. But if you had better idea, feel free to change.

Joker wasn't able to use persona in the real world. It would made him too OP with a giant Satanael wreaking havoc. He can only use the power of the Spirit he had saved in the real world. And he can use both in the Metaverse.