I have not tried adding additional flavors as it's outstanding as is. However, 5% Strawberry Ripe (TFA) or Real Flavors Strawberry Soecial Concentrate (SC) at 1.5 % or Shisha Strawberry at 2.5 % or Medicine Flower at 1% might be really good. You never really know unless you experiment. A five or 10 ml test would be good to start with. I might have to try it myself.
You might already know about this, but this recipe is a good foundation to build upon when your just starting out. Even some of the suggestions from other people's comments spark ideas to try different variations. It's an old favorite of many. I will have to make a variation with Real Flavors Strawberry.