Many of the most common sexual roleplays involve a power differential and form part of the dominance and submission aspect of BDSM.
Roles can be general designations of power positions, or very specific, detailed fantasies.
They're also great role-play inspiration because they blend eroticism and danger: You're the virginal princess, and he's the prince charming trying to rescue you from the evil queen." / Bring the superhero trend into the boudoir: Batman and Catwoman, enemies with a strong sexual tension; or Superman and Lois Lane, who's held hostage by the bad guy.
The best part of this scenario is the costumes—go all out with a skin-tight catsuit, thick black eyeliner, or even a mask.
Some people, such as those living the Gorean lifestyle make use of an entire imaginary world.