Stick with the regular ole Coke Classic. I suppose you could use vanilla coke but it's only four ounces in a flush, why bother.
Coke used to be an herbal formula down south. Of course they took the cocaine out of it years ago and the only thing bad about it now is the high frutose corn syrup. And too much of it will give you too much phosphorus. Which isn't good for calcium levels.
But people have used other ingredients. But again, why bother, it's only four ounces and if you're that picky you still have options, which is a good thing.