I agree with Mike9 on grinding together the meat and the fat when raw. That's my usual approach. And yes, I sometimes use the food processor instead of the grinder.
And while I normally very lightly season the patties and use the toppings and sauces to get the desired flavor, I have certainly ground in onions, garlic, peppers, mushrooms, herbs and such right in with the meat.
It's a burger - the main rule is to make something you like.