Heat oven to 300° F. Spread the bread in a single layer on 2 rimmed baking sheets. Bake, tossing once, until dry, 30 to 40 minutes; transfer to a large bowl. Increase oven temperature to 375° F.
Meanwhile, melt the butter in a large skillet over medium heat. Add the celery, onions, 11/2 teaspoons salt, and 1/2 teaspoon pepper. Cook, stirring occasionally, until the vegetables are tender, 10 to 12 minutes. Stir in the sage. Transfer to the bowl with the bread. Add 1 set of mix-ins ( Bacon, Pecan and Thyme or Pine Nut, Raisin, and Parsley), if desired.
Combine 2 1/2 cups of the broth and the eggs in a medium bowl. Add to the bowl with the bread and vegetables. Let sit, tossing occasionally, until the liquid is absorbed, 5 to 10 minutes. (Add up to 1/2 cup more broth if needed; the stuffing should feel moist but not have any standing liquid in the bottom of the bowl.)
Transfer the stuffing to a buttered 8-inch square baking dish. Cover with foil. Bake until heated through, 30 to 35 minutes. Uncover and bake on the top rack until the top is browned and crisp, 30 to 40 minutes more.