Since then, I've been told that one should use less brown sugar because of the molasses. I've also seen variations which use some measure of white and brown sugar together. And finally, I've always been tempted to just go over the edge and substitute dark brown sugar, but I expect it would just dominate the taste instead of bolstering it. But someday, I'll know for myself. Beyond sugars, though, there is a whole world of natural sweeteners.
Honey is one of the more popular ones, and offers a wide range of subtle variations.
A good starting point is simply substituting the same measure of a generic honey for your sugar syrup, and then vary to taste. But from there, consider using specific kinds of honey. Orange blossom honey can be very different from clover, and other flowers each can add their own special character to your liqueurs. Remember that some liqueurs are made from nothing but flowers, syrup and vodka. Honey offers a special problem in filtration , however, being prone to leaving very fine mists at the bottom of your jar.