Just a few thoughts that I've had recently. Though a bit specific, I figured if anyone would be interested. Player gender does not matter at all, and in most cases I'd be happy taking on either role.
Starcraft - Nova
Starcraft 2 minor spoilers follow~
This scene has a number of possibilities, but it would involve the infamous Ghost named Nova from Starcraft lore. Nova / Tosh, Nova / Stetmann, and Nova / Raynor come to mind as ideas, though I'm up for suggestions. Possibly romantic, possibly nonconsentual. Ideas could range from affectionate to downright brutal. Tosh raping Nova for her interference in his plan, Nova seducing Stetmann for intel, Stetmann performing experiments on a captive Nova, the legendary Ghost 'thanking' Raynor for his assistance, Raynor taking his pent up need for sexual satisfaction out on a captured Nova, etc. Feel free to let your imagination run wild! If you like the character, do feel free to suggest ideas.
Forgotten Realms - Seven Sisters
The Seven Sisters pose a lot of possibilities. Mostly NC / blackmail / etc, though I'm up to ideas. Alustriel, Qilue, and Laeral mostly come to mind. Obviously pre-4e. Possibly an enemy of theirs in the Realms, possibly with a shy admirer who's performed a profound service for them and deserves a reward, possibly with even a modern boy (or girl!) who lusts for a chance at their fantasy vixen and by some means of magic is given the opportunity...
Final Fantasy XI - Mithra
This is perhaps the most open-ended of any of my ideas. Romance, NC, slavery, M/F, M/M, whatever appeals. If any idea strikes your fancy, I'm up to hear about it!
Her part time job is terrible. Her classes are becoming too much to handle. College and living on her own is turning into a disaster, but it's her only hope to continue escaping the abuse suffered at the hands of her step-father. Then, her younger brother finishes high school and decides to attend the same university with flawless grades and enough money in grants to support at least two people. He'd certainly support her without question given how much he adores her, but perhaps there's something she could do to repay him. After using his laptop to browse the net and check up on her long-neglected email, she comes across a few, erotic text stories about herself, written by her own brother. It seems he's had a severe crush on her all along...
Whether blood-relations or step-siblings, the idea is a touch romantic, and a bit deviant. What the sister does with her sudden revelation would depend on how the rp goes!
(Taken, thanks all for the interest!)
On a slight twist to the incest-play, there's always an aunt looking for a place to stay after realizing she has to get away from her husband for whatever reasons.
Warhammer 40k: Adepta Sororitas
Upon an embattled world, one of the Sisters of Battle is cut off from her battle sisters. Trapped within a bunker with only a single guardsman or Space Marine scout to rely upon for mutual survival, the pair have to depend upon each other to survive as death and destruction engulfs the planet. Perhaps, the tide might still be turned. And in times of need, perhaps a deeper bond is forged.
I'm also up for suggestions. Do read the O/O and send a PM if interested. :)