Bulking Up Ground Meats

I have recently started adding a few things to my ground meats - venison, chicken/turkey, and beef, haven't tried it with pork yet - to add more nutrients, flavor and bulk (making it stretch a bit farther). Here are some things that I've added and a few that people have suggested (**). This is per pound of meat:

• 1 cup finely shredded carrot
• 1 cup  finely chopped fresh spinach leaves
• 1/2 cup finely shredded zucchini
• 1/2 cup finely chopped onion
• 1/2 cup finely chopped bell pepper
• 1 clove minced garlic
• 2-4 tablespoons ground flaxseed
• 1/2 refried beans** (I'd opt for making homemade beans and then pureeing them)
• grains such as cooked rice, cooked barley, uncooked oats, bran cereal, crushed crackers or breadcrumbs
• finely chopped mushroom**