Tried for this line a few years back and it never came to fruition, so I want to give it another shot.
Character A is a djinn/genie who has been bound in servitude for centuries. Many former masters, all of them greedy and sometimes outright cruel, has left him with nothing but disdain for the utter selfishness of the entire human race and a grim sort of resignation to his fate. Over the years he has convinced himself that all humans are the same; they want what they want and will use him by any means necessary to get it. He does, however, ironically take a kind of pleasure in the few short days at a time that he's free of his confinement whenever a new master summons him, because unless he is bound to a master, he has no physical existence and cannot feel or touch or taste, so he treasures every moment he is allowed a physical body to enjoy those things.
He then meets B, his newest and most recent master, and B does not follow the script. B is not selfish or cruel. B is used to doing things for himself, is kindhearted in nature, and has no interest in forcing A to do anything against his will (even though A has little choice in that respect; the genie rules prevent him from going against his master's wishes). In spite of his aversion to humans in general A finds himself falling hard for B, for his good and kind nature and even his independence. But the longer they put off using B's wishes, the more A finds he is tethered to the mortal realm and becomes more human in nature. Which would be fine, except that they have no idea if there is in fact a time limit on wishes or what happens to A if his time runs out. Then it's a race against time to learn the truth (possibly with a new, or very old, enemy thrown into the mix), and a heartbreaking choice is before them: use B's wishes, after which A will disappear from his life forever, or let the time run out and risk everything in doing so.
Looking for B! This would be a slash line, obviously.