I think the idea of focusing on a single actiony quest is a good idea, though I'm not a huge fan of it all taking place underground. Aesthetically speaking I'm not actually super big on dungeons/caves, at least not to the point of spending an entire game solely inside one.<br/><br/>Maybe we could do a Demon's Souls set up? Where they get tied to a sort of nexus area that lets them venture out into different areas of the world to gather up the strength/artifacts they need to fight the big boss? It's the same idea more or less, but a bit more variety in locations. Plus it helps with the central hub idea where they can send NPCs they find back to the nexus and build up a tiny settlement.