1.) Modern Tarzan. Character A (me) has been lost to the wilderness/jungle since an early age after a plane crash stranded him/her (I think that this line would be much more interesting with a female Tarzan) and killed his/her wealthy parents. There have been rumors of a wild man/woman that have fallen into local legend. Character B can be an anthropologist on what is considered a wild goose chase until it yields surprising results, a similarly stranded party who stumbles across the wild man/woman, a "bystander" to the expedition in search for him/her such as a cameraman or really anyone who has reason to find and "rescue" him/her back to civilization. I'd like to focus on the trust aspects and the difficulty of transition to a modern life along with it being the first relationship for my character as he/she becomes civilized again. This one could be het, femme or slash and can definitely go dark with issues of trust and power.
2.) Age gap. I'd like to play the younger man or woman in a slash or femme line against an older PB. Ideally, I'd like it to be less about any sort of taboo that might come from something like that and more about making seemingly opposing lives work together. A mature doctor vs. a free spirit local musician, for instance.
3.) My character is an alien crash-landed on Earth. Though able to take human form, she is captured by the government at the crash site. Your character could be a scientist with evil motives all in the name of science, a soldier with pure ones to help her escape or something else we can come up with.
4.) Vampires. I'd like something similar to the plot of Only Lovers Left Alive; a man (me) and his lover have been off and on over the centuries and they are reunited again after some time apart. Ideally, I'd like your character to have made mine into a vampire and focus on their interactions with each other along with his weary view of the ever-changing world around them and your character's slightly more, ahem, sunny disposition in comparison. Slash preferred.