I can think of dozens of varieties in which this RP theme could be played, each unique. To come up with plots will be easy, and I'd gladly include your preferences. The theme is also perfect to base vignettes on it. The story's focus can alternate between the real world and the dreams, or mostly ignore the real world. A longer RP (more than a one-scene vignette) can have extreme differences in its mood and themes from dream scene to dream scene.
The general idea:Your character finds, knows, or is revealed a means to create dreams exactly as he wants them. These dreams, he can let my character experience. He can enter these dreams in whatever form or appearance he chooses, actively participate in them, control all he himself does, and most of what happens. He cannot, however, control what my character thinks, feels or does other than through his actions or the situations he creates (ergo: as in any other RP situation).
For their duration, my character experiences the dreams as if they were her reality, there might even be a slightly increased sensory and emotional intensity, and a reduced self-restraint, as dreams are closely connected to the subconscious. Your character would be awake, fully aware of having sent his mind inside a dream he created or manipulates, but apart from this knowledge, everything within the dream feels perfectly real to him.
Your character can choose to create dreams or nightmares with all the themes and scenarios at all imaginable, have a purely scientific interest for these dream studies, a motive of revenge or desire, or use them as a means to do the forbidden, explore the possibilities, to do whatever he wants inside a world he can enter and leave at will. Your character can decide what mine will dream, what of it, if anything, she will remember. Inside the dreams, he can indulge in whatever he wants without suffering any consequences in the real world (unless, perhaps, he should have pangs of a bad conscience or dislike his memories of the dreams, or develop an obsessive addiction for them, etc. - that would be your choice). The dream scenes can involve whatever elements and themes we want, range from vanilla over BDSM and NC to extreme.
In a longer RP, I'd like to explore the dreams' development, see how they will affect our characters' personalities and lives, or possible 'real world' interactions between them (if we want to focus on these).
This RP can focus heavily on sexual themes, and include NC, BDSM, and Extreme.
Caveat: You should be fine with a slow pace, and write a minimum of two paragraphs per post. I prefer descriptive, detailled writing, emotional and psychological aspects interest me, I enjoy partners who can create an atmosphere, write credible, convincing characters, and seek those who would like to explore the effects the (ab)use of one character's (near absolute) power and control over the dreams of the other (and indirectly, their psyche, possibly even their lives) will have both on the manipulator and creator of these dreams, and the dreamer.