The Old Fashioned is a bold, booze forward drink, showcasing American Bourbon, in a restrained, balanced way, served over a large piece of, preferably, clear ice in the eponymous glassware.
If we think of the Old Fashioned as a template of spirit + sweetener + bitters, then we can extend the family of drinks by swapping out any of these ingredients.
Swap bourbon → vermouth: Vermouth Cocktail.
Swap bourbon → rye and sugar → maraschino liqueur: Fancy-Free.
Swap bourbon → rye and sugar → Benedictine: Monte Carlo.
The spirit is driving the flavor in the Old Fashioned family. But we must balance all that heat...
The sweetener is used in small amounts to balance the "hot" taste of the spirit. But things would be flat if we stopped there...
The bitters are used to expand the flavor and pair with the flavors in the spirit of the drink.
Each ingredient supports and expands each other ingredient.