How about you try freezing as is? Then depending on the use you could mix it with something or leave it as is.
You might find that you want to try something different later on, say bacon as apposed to beef fat. Also you might want to make jerky, chili, stew or some other recepie where plain ground deer is best.
Actually, I never mix anything in it unless I am making sausage. It doesn't need anything else to be good, but if you want some variety, prefer a fattier burger or need to strech out your deer meat mixing something else into it might make sense.
Juniper Bow