Design a duty?

Probably make it feel like an actual duty instead of little more than a single level of a side scrolling game.

Branching paths with doors that open via puzzles or retrieved items. Make the puzzle solutions generate randomly each time, Randomly place key items each time, Add traps that do things like add hindering zone effects, debuff players, spawn enemies or cause them to respawn faster, or buff enemies.

On that note have enemies constantly spawn and respawn. Set condition that increases spawn/respawn speed based on number of enemies engaged. Sub bosses randomly spawn from a possible pool within duty and create an engagement zone when aggro'd. Door to main boss doesnt open until atleast 1 is defeated. However gains special abilities and stat boosts for each sub boss not defeated on top of being able to summon that sub boss during the fight.

As for a theme? I dunno... maybe a palace full of scantly clad monster girls just to be as visually distracting as possible.