Personally, I like only a 10% fat addition(and venison fat if possible....then wild boar, but pork works just fine too). One of my favorites is a bleu cheese apple with caramelized onions I created in culinary school. I made a large batch but here is the recipe I had worked out.

15 pounds meat cubed
1.5 pounds fatback cubed
10 apples
5 onions
3/4 poundsbleu cheese crumbled
1/4 cup veg oil
4 T smoked hot paprika
2 T dark chili powder
3 T ground coriander
1 T ground cumin
1 gallon apple cider vinegar
Saltand Pepperto Taste

In a large pot add the oil and put on high. Dice or slice the onions and add to the oil, cook till darkly caramelized stirring occassionally(note that stirring stops caramelization from happening so don't stir too often, just enough that you don't burn the onions). When the onions are caramelized add the gallon of cider vinegar to deglaze and allow it to reduce down to a syrup, approximately 2-3 cups of liquid.
Peel and core your apples. Grind meat, fat, spices,and apples with a 1/8" dye or a medium dye if you prefer. mix in the onions and the cheese thoroughly. create a small tester patty and cook it off to check the salt and pepper level and adjust accordingly, repeating as necessary. When the seasoning is done, stuffyour sausage and enjoy.