The harem should not be a monolith of cheerleaders. Mix righteous paladins with manipulative demonesses. This forces the protagonist to constantly negotiate his moral alignment.
Harem fantasy stories often deal with world-ending stakes, but a growing trend in light novels, manga, and web novels is subverting the traditional moral binary. The classic trope of a pure, chosen hero defeating an explicitly evil demon king is being replaced by a more nuanced narrative mechanism: the "Good or Evil Will Save the World" fix. This concept forces the protagonist and their harem to navigate a world where the forces of light are corrupt, the forces of darkness are misunderstood, or the survival of the universe requires balancing both sides rather than destroying one. harem fantasy good or evil will save the world fix