Fire Emblem and Persona Series Crossover
I do not own neither one, They are Respectively owned by Nintendo and ATLUS
This is a summary of an idea fanfic for writers, not a actual story.
An Idea when I played Persona 3, 4, and Q and Fire Emblem: Fates.
As the prologue in the game of Fire Emblem: Fates starts off so would the climax of the Persona Series of 3 and 4 as the SEES face off Nyx and the Investigation Team fights Izanami.
As separate groups begins to fall one by one, the P3/P4 protagonists hear a song ringing and echoing into their heads as their vision fades out to white.
This is where they enter the FE: Fates world.
Starting off is some of the SEES character awakes in the middle of the dark forest of Nohr and then were capture and brought before the Nohrian King. After a little quarrel between the royalty with a certain prince named Corrin, he propose to make the P3 protagonist and his friends as acting retainers, even if the King decides to split his friends up with the other siblings.
Then when Corrin and the P3 protagonist were sent on a mission, they both were split up as Corrin disappears.
Afterwards, when Corrin came to, he notices the P4 protagonist and the rest of the Investigation Team and then were escorted to the Kingdom of Hoshido where they were welcomed and offered sanctuary as they stuck themselves with the Princes and Princess until they find their way back home, even though their name style is similar to each other.
And then when the incident in the Kingdom's plaza and the death of the Ruler of Hoshido has happened, this placed Corrin in a tough decision where the Investigation Team offered to help in a way in order to repay the kindness the Hoshidan gave to them.
While the High Prince left to engage the invading Nohrians, Corrin and the P4 Protagonist decides to follow, where they meet up with the rest of the royalty with the P3 protagonist in toll.
And thus, this is where Corrin had to pick a side. Fight for Nohr with the P3 protagonist as his retainer or help defend Hoshido with the P4 protagonist as his retainer.
As for the Persona users of the opposing side, the Persona protagonist may chooses to serve the High Prince as their retainers, and thus their journeys began to intertwine as their respective stories of Birthright and Conquest progresses.
Regarding the third option in the game, Corrin could choose the option with the Persona Protagonist understanding his reason and support his wish, exclaiming "We are Corrin's retainers! We will not allow you to force him into a decision he may regret. Even if you are his relatives!" ... something like that.
Although they may have their persona abilities, they weren't accustomed to combat against actual people as they are to Shadows. Perhaps even the Shadows would take form of humanoid units but with Shadow masks.
If I were to consider the Persona User's class they'll take in the game, I would like to suggest some...
Minato as a Mercenary to a Hero. He wields a single-hand sword much like some of the Mercenaries in the FE games, he just has a little more armor on him with a shield-like shoulder armor.
Yu as a Samurai to a Swordmaster. Well... yeah... he wields a two-handed Sword/Katana. He dons some armor pieces that represents his class.
Seeing how the two classes counterpart each other in Fates, that could work in a way. Maybe even the royal sibling from each side could offer them a unique weapon that suits them.
Now there's two notable characters that I hope to mention and that is about Akihiko and Chie, The Boxer and the Martial Artist don't exactly use or wield any weapons. So seeing that this is a fanfic, I think it's fine to "make-up" a class that represent them, called the "Pugilist" Class that uses Gauntlets and maybe Greaves. Due to their alliance to their respective kingdom, I'd say their class could promote themselves to the class that goes with the Kingdom they ally themselves to.
Akihiko, initially a Pugilist to a Nohrian Bruiser, Fighter who use their fist to pummel down their foes. Equivalent to Fighter units by their High HP and Attack output.
Chie, Initially a Pugilist to a Hoshidan Monk, Fighters who follows their teachings through their practice of Mind and Body and their tranquil surroundings. Able to wield Staffs to heal.
Seeing how they had no where to start looking for their way home, they would support their respective armies in a way that would be their best bet for survival. Otherwise, those in the Velvet Room will tell them that certain requirements are need in order for them to return to their world, like in Persona Q where they had to be those Giant Shadows in the labyrinth.
And for the Velvet Room, It would just appear in Corrin's My Castle where a small fortune-telling booth would automatically appear. While it look so small, it's actually a huge room on the inside.
If there's that possibility... There's a unique part where the game offers this, marriage and childrens. Seeing how the father, not the mother, that determines which child is born, could that possibility happen to the Persona Users? But then again, they're aren't at any interest to bear any children, at the time and even sending them to the Deeprealms seems concerning, should they learn more of it, especially the accelerated time flow of the Deeprealms. Maybe instead, the Persona User's children would make an appearance from traveling from their world's future timeline due to their Parent's "Disappearance" so they'd band together and investigate this themselves, bringing them to the world of Fates, seeing their parents, the Persona Users, at their prime of youth.
If anyone is interested in adopting this idea, your free to do so, I'd like to see your writing into fruition.