Over the years that I've been doing IM-style roleplays, these setups have worked out very well. Now that I'm here, I'll include some other setups which are more complex and probably better suited for play by post, where paragraphs of plot detail can be worked out without someone waiting for you on the other end in real time.
Going Back to Camp: (M/F, M/F/F, incest, can be NC):
You and I divorced several years ago. We married young, and tended to the stupid things young people do. We had one daughter. You got sole custody of her, and have raised her as well as you can. We communicate very rarely, and haven't spoken in the last two years.
You and I used to go to a special camp for two weeks out of the year, every year, when we were married. This camp had special rules and ideas about how people would act (and be allowed to act) while you were visiting. Now that our daughter has come of age, you look at her and remember me, and us, and feel that yearn in your heart again for the times we were together at the camp. After struggling with yourself for a while, you realize you want me back. You take a chance and tell our daughter that you have signed the two of you up for a week away at this camp, but don't tell her about what else goes on there other than hanging out by the lake and horseback riding. She's excited, for certain, and you are too, for different reasons.
On your first day there, you're both strolling through the grounds when, passing a cabin, you look up and see me, standing on the porch, wearing an old t-shirt from a concert that you gave me when we were first dating.
Aboard Ship (M/F, M/F/F, possible incest angles, possible NC):
You and I were lovers, many years ago, and explored kinks together that would make others shudder to think about. Our lives took us separate directions, and we drifted, like many people do. Older now, both with other families, we happen to both take the same cruise together, along with our respective spouses and nearing adulthood kids. Seeing each other again after so long sparks something within each other. What happens next takes both of our families in directions they never dreamt of.
Soccer Team in Las Vegas (M/F, M/F/F, incest, NC?):
I coach my daughter's school soccer team. You're the mom of another daughter, also on the team, though we are not married to each other. I see you at games a lot, you help out with setting things up and arranging carpools and so on. The team does well, and gets invited to go to Las Vegas for a championship series. You agree to come with the team on the trip to chaperon. On our first night there, after the girls are settled into their rooms, you and I go down to the hotel bar for a drink. Over the course of our conversation, we discover a hidden, mutual lust for the girls on the team, and over the next few drinks, decide you and I both need to do something about it.
New Friends At School (M/F, incest, NC):
Going to a new school is always a delicate time for any girl. It helps to have a friend, and when you meet Alana, you think you've found the best friend in the world. She understands you in ways no one else does. In the end, it's you who first kiss her, and that kiss leads to some light other petting, and you finally understand that you're a lesbian. You have to tell the family. When you get up the courage to blurt it out at dinner, it doesn't go well, and soon Daddy is yelling and Mom is weeping. You dash a retreat up to your room, also crying. Later that night, as a dutiful father, I come upstairs to try and work out what happened to you, my girl, and with the determination that I can change you back, whatever it takes.
"I know what you've been doing, Daddy..." (M/F, incest, table-turning, NC???)
You suspected I was having an affair behind your mother's back, but you didn't expect to see coming out of a hotel downtown with your best friend from high school. You know if your mother found out, it would devastate her and tear the family apart, and you're not about to let that happen. In fact, you're prepared to do anything, anything your father wants in order to get him to stop his philandering.
The Resort (M/F, M/F/F, play within a play)
A private island in the Caribbean caters to the extreme tastes of some of the wealthiest men on Earth. Nothing is forbidden, no request unfulfilled. I arrive there, looking forward to my time, but find more in the way of relationships and strange attractions than I bargained for.
(This one can go in a lot of different directions, obviously, and will have some roleplaying within the roleplay. :) )
Superheroine Transformation (M/F, M/F/F?, incest?, NC)
You are a Supergirl / Powergirl type superheroine, able to leap tall buildings within a couple of bounds, and fly, and smash through things, and do all the things that superheroines do. I am one of your villainous nemeses, and we've tussled before though I've always escaped. I don't have superpowers but have a huge intellect and invent many devices which give me somewhat exaggerated physical abilities. At least until you smash them.
Then that fateful day happens. Your normal human sister calls you on your cell phone, begging you to help, to rescue her, as she's been taken by people she doesn't know. The line goes briefly dead, then a voice tells you to go to a warehouse downtown if you ever want to see her alive again...
(This scene can go in a lot of directions, but mostly they lead to the sexual subjugation of the superheroine by the villain, and him turning her into his sexual pet, making her into a part time supervillain herself. Her sister may or may not get in on this action as well. You don't have to be that particular superheroine if you prefer one with different attributes or powers.)
Sex, Magic and Rock & Roll (Potterverse [no canon characters] m/f m/f/f dark magic and who knows what all):
This one is a little less fleshed out than the others but having just re-read the Harry Potter series, I'm inspired to do some RPing in that world. A wizarding rock and roll band tours the world, but in their wake dark magic seems to erupt. A female Auror is sent to go undercover and investigate the band, only to become steeped and seduced by its secrets.
These next two are more suitable for a longer story line.
Adventures in Ardai (fantasy, M/F, M/F/F, F/F, NC, corruption, incest?):
Ardai is a city on the river Mels, and has over a hundred thousand people living in it. Built over many hundreds of years, Ardai's underbelly contains many sewers, hidden dungeons, and other places for thieves to hide things and for evil to lurk. Senda is the major goddess of the city, and her priesthood provides healing for the sick and wounded of the city. Temples and enclaves dedicated to Senda dot the city. You run one of the smaller ones dedicated to bringing up girls into the priesthood, like a cross between an orphanage and a boarding school. You were raised there from the time you were eight years old, and know nearly ever girl there very well. Many are your childhood friends.
A thousand years ago, the army of Ardai, aided by the priestesses of Senda, were at war with the neighboring city Vethana, whose inhabitants were in thrall to the dark god Thrux. It was whispered that terrible rites were practiced by Thrux's cultists, involving perverse sexualities only whispered between war-weary troops on cold foggy nights. The Ardaian army crushed Vethana's city walls and wiped it from the earth. The stories and legends of Thrux drifted from real threat to legend, and people now barely remember that such things even existed.
That is, until your best friend goes missing, and you search after her, only to discover terrible truths.
The Imperial Assassination League (Star Wars, M/F, M/F/F, F/F, potential incest, NC, and hot twi'leks):
Twenty years after the Emperor's ascension, the remaining Jedi cower in fear of the Imperial armies and black squadrons. The Death Star has been constructed and destroyed, and the Rebellion is throwing all into chaos.
With the Imperial regulars distracted with chasing leads of the Rebels, Coruscant becomes relatively uncontrolled, and in the warrens and sewers of Below Ground, a small group of female Jedi and potentials find each other, and hide. Their mentor, the Twi'lek Ardana t'Racen, struggles each day to find food and scrounge credits for medical supplies, as one of their company, a young Jedi with huge potential, has an illness that keeps her close to the pharmaceutical factories in the fourth district.
Ardana is assisted by the human rogue Limna Bet, She pilots cargo vessels, and smuggles on the side for the Rebellion. Limna and Ardana have a history, but their romance has been dormant for a couple of years now as events have overtaken the both of them.
It is then that Imperial Commander Maltus Neerien discovers Ardana and her little band. Rather than turning them over to the authorities, Maltus uses the girls for his own gains, and his own pleasures.