How can I tweak this (US) Southern themed cocktail?
How about some thinly-sliced peaches? Seems relevant to the Southern theme and complementary to both the heavier bourbon flavor and the other fruity notes.

You could sub out the agave for peach syrup if you want to live on the edge.

Rhubarb is also a Southern classic that could be an interesting addition. But I don't know how you'd implement that.