Sassafras tea concentrate, vanilla extract, lemon juice... Here's a recipe I put together recently I like.
4.5 lbs sugar
1/4 cup of lemon juice
1 lb lactose
1/4 cup vanilla extract
2 oz root beer concentrate
6 oz sassafras concentrate, it isn't that concentrated
Sugar, lemon juice and a little water in a pan. Get a couple of quarts of water ready. Cook sugar in the pan until it's medium caramel. Add the 2 quarts water. Fully dissolve sugar. Add lactose, fully dissolve. Add vanilla, root beer concentrate, and sassafras concentrate. Mix thoroughly. Add to keg. Add water to 5 gallon volume in the keg.