You know Pokémon Conquest? The Pokémon Tactics game crossover with Nobunaga’s Ambition? Elevator Pitch. Same idea, but with Fire Emblem. And you pull a Kingdom Hearts with legacy FE characters all being recognizable but being non-canon to their original entries and that way you can have Marth, Ike, Roy, Robin, Corrin, Chrom, Lucina, and Byleth all in a game together without crossing dimensions.
Edit: And just to be clear, this would be clear, this would be Fire Emblem x Pokémon. Not Fire Emblem x Nobunaga’s Ambition.