Writing ideas come from funny places. Take my current project:
The setting is courtesy of a dream I had. This wasn't the setting for the dream itself; it was the setting for a MMORPG that I was play(test)ing in the dream. But it was a hell of a cool setting, and I was having a lot of fun playing a character there - or dreaming that I was playing, anyway. (It's really not as surreal as it sounds...) If I were a programmer, I might try to build that game. If I were an artist, I'd draw pictures of that place. (Unfortunately, stick figures really wouldn't do it justice, and that's about the limit of my artistic ability.) As it happens, I enjoy writing, so I decided to set a story there.
Which was fine, but what would the main character do? Well, he'd be a new arrival; so there would be scavenging, exploring, and other survival-oriented activities. He would discover that people here had powers - they could do things that weren't supposed to be possible. And he'd quickly discover that some powers could be acquired... and not just acquired, but accumulated. It was dangerous, but it could be done.
What sort of power would he acquire? What direction would he go with that knowledge? In a setting like that, what would he want for himself?
I read a lot of comics when I was younger. (I still read some, but it's nowhere near as many. Partly that's because of time and money considerations, and that's partly because the quality seems to have dropped off - couple not-so-good writing with an addiction to endless crossovers, and I'm gone.) At one point, while reading the New Mutants, I started wondering about what sort of character I'd add if I were writing a story in that setting. Well, Marvel Comics has a character called Selene, a sort of psychic vampire who drains people's life energy to fuel her powers and extend her life. And it occurred to me that if she had a son, he might have a similar power, but fueled by blood. This appealed to me, since it added an interesting moral dimension to using his power: he could be as powerful as he wanted, if he was willing to take enough blood. Plus, it would be fun to write: you could introduce him with a couple of minor powers (strength, dexterity, a bit of damage resistance, and a powerful mind shield to hide his secrets from prying telepaths), and then drop hints (dead animals around the estate, using powers he hadn't shows previously). When his parentage and the true nature of his powers was finally revealed, you could have a big, dramatic showdown over whether he'd be able to stay with the team.
Sadly, Marvel never hired me as a writer, and the idea was left to languish on the top shelf in the back of some dark closet in my mind. I didn't forget entirely, though; and it occurred to that it would be an excellent fit for this setting.
The character himself wouldn't work, though. His background was too specific to the Marvel Universe. That could be adapted, but I didn't want him to be born with the power. I didn't want him to be born in the city at all; part of the weirdness of the setting is that people just occasionally wake up there. (New arrivals have no memory of their past, or how they got there; but they retain their skills, and they all have a single power: they can understand each other's languages.)
Which led me to another question: what sort of person would actually want a power like that?
It would be somebody who wanted access to a lot of power, obviously, and wasn't too picky about what he had to do get it. So, probably somebody who didn't have a lot of power, and couldn't afford to build up slowly (by accumulating minor powers). Right about then, my protagonist sudden became female, and Eve was born.
Eve, I thought, would be a new arrival and something of a loner by nature. She'd be a fighter, but also realistic about her limitations. She would feel trapped by the people who had taken her in, and by her inability to survive without them. Then she'd learn about the Temple on the Hill, and see a possible way out - if she could survive it.
Eight weeks and fourteen pages later, I have the heart of the story. It already begs for a sequel.
I wish I had more time to write.