In my wife's family putting peas in it is my a time-honored tradition. They even sing a little song: "Eat that mackie cheese and peas," or "Eat that mackie cheese, no peas," depending. Especially camping - they use dried peas which get rehydrated by the time it's cooked. The one additive they also like that I can't stand is tuna. In a sandwich, great, but not in my mackie cheese pleeeze.
Peas (fresh or frozen) are nice and bland without being gross about it. You can add them to all kinds of stuff and forget they're even in it. Soup, pot pies, mashed potatoes, anything with gravy or cheese, meatloaf, etc.