Not legendary, but I'd love to see Senran Kagura x Soulsborne, with some tweaks.
>pick only one of the girls for a run, the rest have now been turned into horrific mutations, demons and whatnot
>the city has turned to shit, so familiar Shinobi enemies are now undead, and more vicious, like an undead bear Shinobi or the fucking crossbow maid ones
>artstyle is still anime, but much more bleak and detailed in gore, grime and dirt
>girls trying to find their way out, stay alive or just do SOMETHING
>the other girls you didn't choose have now mutated according to their animal and weapons, so that makes it ice spider Yumi, Minori surrounded by rotten cake and food, yagyuu as a huge squid, undead hibari riding her pet rabbit with burnt skin and emmiting thunder, blob molten haruka, mirai with all her firearms fused into her skin, etc
>game is shorter than a Soulsborne because the city isn't that big, but would be more focused on boss fights, and sometimes unexpected fights or short skirmishes against them; gameplay is similar to Soulsborne games, maybe a little bit faster