Several years ago, I was taken to a jiaozi restaurant in Beijing. This place had literally hundreds of different jiaozi. Every kind of meat (some decidedly endangered and illegal), seafood, vegetable etc. They even had strawberry jam jiaozi and ice cream jiaozil. I forget now what I actually ate.
Of those I've made myself, the favourite filling was a 50:50 mix of pork and the local blood sausage.