So I was speaking with my GM about it yesterday and he seemed rather dissapointed that there were only 4 listed diseases. There was the joke about 7 being more appropriate but the limited number is still rather, well, limiting.

I thought perhaps as a community we might come together to craft some new diseases for our Father?

Warhammer Fantasy 2nd edition (in the core book and the Chaos-themed Tome of Corruption) had a pretty extensive list of plagues/diseases and their extended prognoses. It might be worth just lifting from those (and making characteristic damage recovery from diseases impossible until the disease has ended or been cured).