I'm not sure exactly how you're making the soda, but one flavor I'm totally hooked on is made with lemongrass simple syrup. I take a few stalks of lemongrass, cut them up into two inch chunks, then blend them in my Vitamix with a cup of water. Afterwards I add a cup of sugar and simmer the stuff for about 30 minutes. Then I pour it through a tea towel into a jar to filter out the grass, let cool, then store it in the fridge for soda making. I would experiment with it, mixing with a little selzer water (so you'll know what its like carbonated), until you have a good balance to your taste. Afterwards make it in a soda recipe you have.
I don't like adding anything other than the lemongrass SS and what's necessary for yeast-based carbonated soda.