I used to write OC romances more often than not because I honestly couldn't imagine pairing up the Doctor, or Snape, or whoever, with anyone we'd so far met in canon. So part of the challenge was imagining somebody who would make a good match. And then, of course, winning the reader over to liking the OC and believing in them as much as the canonical characters, which was a whole other challenge in itself.

That being said, I read a great Mycroft/Anthea fic once. I wish I could remember who had written it...