I do this a lot, to be completely honest. I think as writers, we're all inclined to be creative at even the smallest spark of inspiration, whatever the source. So even if we see a thread we've no interest of joining for whatever reasons, be it time or people or direction of story, something very small may plant a seed in our head the becomes a character. I'm sure we've all even made characters based on ideas we didn't even know had come around. My character Rayna for example, I later concluded, had been inspired by the way a previous RP (I read, but never joined) used rain as a major plot device. I of course didn't have this in my conscious mind at the time, but looking back on her name, her powers, and her appearance, all of them can be attributed in some way to the rain in the RP I had scanned over.