Hmmm. As someone who loves to write rarepairs, number 3 really resonated with me. I think they are more of a challenge, particularly if you're writing a first-time fic. In a text-based fandom like HP, it can be because sometimes you don't know anything about the character beyond a mention in the text somewhere, so your challenge is to make a believeable, consistent character from a name and (hopefully) a few other details. I once wrote Ignatius Prewett/Antonin Dolohov for an exchange, and it was the most incredibly rewarding of fics. Sometimes, though, the problem is not one of inadequate canon characterisation, but, as you point out, plot complexities rising from the problem of how your characters meet at all. This can be really satisfying too, although I must admit to a shitload of canon fuckery (MCR's tour schedule, for example) at times to make an idea work.

As a writer, too, I like the idea of offering people something new, something they may never have considered before. I know that doesn't always make for the biggest readership, but I don't really care. It's really satisfying to get comments - even if it's only a few of them - from people who have never read your pairing before, or thought that your pairing would never work.

I think rarepairs can be incredibly satisfying, and I quite like this theory. I know you wrote it from the POV of a reader, but I think it holds true well enough for writers too. What do you think?