Honestly Losttale sounds interesting, but here are two of my own suggestions:

A) Multiple humans fall underground, and have to work together to return to the surface. Along the way, multiple monsters agree to help them, though there are plenty that want nothing more than their souls.

B) Post-game where with no barrier, monsters and humans are forced to coexist once more. Tensions between humans and monsters quickly rise, and history seems bound to repeat itself.