I'd plead the fifth, if ever asked about it, but in a fit of procrastination I jotted down some outline for a Ace Attorney/3 Houses crossover:
(Warning: Includes a few spoilers for both Fire Emblem: 3 Houses and Phoenix Wright: Ace Attorney
It follows the classic 5 case structure of most Ace Attorney games:
Ace Attorney Byleth' father, Jeralt is murdered, and they have to run their law firm overnight alone. Also your father has hired two assistants, Caspar and lazy, narcoleptic but talented Linhardt.
In their first case, former teacher Seteth orders you to defend Flayn, his daughter, who he has never mentioned before and who - so he claims repeatedly - would only harm a fish.
Arriving at court with no time to prepare, you learn, that Flayn is suspected to have killed Jeralt.
The first case usually serves as some sort of tutorial, and has an comical prosecutor, so: Alois (or Ralph or Hanneman).
Hilda, a rich heiress, tricks you to defend Claude for free.
He behaves very suspicious, but it turns out he willingly became the defendant to lure out the real murderer.
(needs victim and murderer)
Edelgard is supected of murdering a corrupt, high ranking clergy man, who did very bad things to kids. She insists, he deserved to die, but she did not do it, but has no alibi. Prosecutor: Ferdinand von Aegir
Murderer: Hubert (He did it, because he knew El would murder the victim eventually herself, so he saved her from that fate)
Dedue sells everything he has to have you defend Dimitri, who appears quite insane.
Prosecutor: Constance (alternatively Ingrid or Felix)
Murderer: Cornelia (?)
Victim: the tragedy of Duscur(?)
Defendant: Rhea, who clearly has several secrets
In the end TWSITD are uncovered, but you decide to keep Rheas secrets.
Cyril and Catherine: admit to the crime to protect Rhea
Prosecutor: Solon/Thales (as with the first Ace Attorney game, the Prosecutor is also the murderer)
Leonie: Your estranged stepsister, a talented policewoman, who in the end helps you.
Ralph: bumbling policeman (compare Dick Gumshoe, alternatively: Alois)
Manuela: Drunk witness (compare Wendy Oldbag)
Sothis: Judge & Jury