Since sweetness is bourbon's central quality, sugar can be a great launching point for riffs when experimenting with bourbon cocktails. As such, maple syrup is a great choice.
The syrup's rich, caramel and nutty flavor pairs perfectly with bourbon. It offers far more complexity than regular sugar and even lends the drink a silky, buttery undertone.
Since both derive flavor from wood, they complement each other naturally. Plus, maple syrup already comes in liquid form, so it won't require additional prep for the shaker.