You could add cooked and cubed ham, spinach, green onions, bacon, more cheese...etc.
1/2 of a large onion, chopped
2 garlic cloved, minced
1/2 cup butter
1/2 cup flour
1 tsp. dry mustard
2 tsp. salt
1/4 tsp, pepper
3 cups milk
4 cups shredded chedder cheese
12 cups potatoes, sliced
In a skillet over medium heat, saute onion and garlic in butter; blend in flour mustard, salt and pepper. Gradually add milk, stirring constantly. Cook until thick. While mixture is cooking, par-boil the potatoes. Add the cheese to the mixture and stir until melted. Remove from heat; add potatoes and toss to coat. Pour into a greased 13''x9'' pan. Bake uncovered at 350 degrees for 30 to 45 minutes, until hot and bubbly.