It is my impression that there is already software out there that has a doctor put in a bunch of symptoms, and then outputs an ordered list of potential diagnoses (including rare ones). The main problem being that adoption is slow. Unfortunately, after 10 minutes of searching, I'm completely failing to find a reference, so who knows how well it works (I know I read about it in The Economist, but that's it).
If it's worth saying, but not worth its own post, even in Discussion, it goes here.