I wonder if this has ever been considered? I think it would be a great idea! imagine the character pool :O and I reckon it would sell like crazy!
As a Musou fan this would be like a dream come true! combining two of my favourite franchises together, SE is already working with Koei for a Dragon Quest style musou game (even though they're saying it's an adventure game) imagine if they gave the go ahead for a Final Fantasy Musou :O