MC trapped in (author's choice: VR Sim, Alt World Dungeons, other similar setting). Cannot leave until/unless able to defeat/bypass 7 different zones/levels/dungeons where everything in each, including the boss, is related in some manner to the 7 Deadly Sins: Pride, Greed, Wrath, Envy, Sloth, Lust, and Gluttony. Theme shouldn't be explicitly stated but subtle clues should be enough for majority of readers/MC to make the connection as the story progresses.