I do not like adding much to burger while I grind. Too often they do not freeze well and it's easy to add afterward with fresh. And it takes away from your ability to be flexible with meals. Sometimes, as with meat balls, I like to grind ingredients into the meat. So I am always setting aside a reasonable amount of course ground burger with nothing at all added - not even fat! Then later as I am fixing a meal I can add whatever is needed and fine grind just before cooking. This also allows me to fluff large grind for Asian dishes or stroganoff

A few nights ago I fluffed some course grind for use in some goulash...oh so much better

We always have three full size freezers full. And since pork doesn't freeze as well as beef fat I don't often use pork fat or pork butt. However, this years deer and the wife's last elk were so sweet and scrumptious I ground with 10% applewood smoked bacon. Burger is now our favorite cut of meat

However, if I were to do it again I would just course grind and add the bacon later. Freezes much better!
Our favorite add-in ingredients would probably be red onion, roasted garlic, gorgonzola cheese and teriyaki sauce. Not all at once of course. And, then with all of these if we do not yet have fat added...it's applewood smoked bacon all the way baby!
Tip: Emeril's red onion Swedish Meatball recipe is to die for. You can make them ahead of time and freeze for an upcoming get-together!