How can I add flavor to meatballs which taste stale from being frozen too long?
Whenever I have freezer burnt or really old frozen meat, my go-to solution is to use my slow cooker. Find a recipe that sounds appetizing to you and has a long cooking time (6-9+ hours on Low), and go for it. For example, here's a Meatball and Vegetable Soup recipe (which I'm not endorsing, it's purely as a "this is the style you're looking for").