See my post here:
http://www.root-beer.org/modules.php?name=Forums&file=viewtopic&t=134
Natural ingredients usually include essential oils and chemicals isolated from essential oils by fractional distillation. Almost all commercial rootbeers contain methyl salicylate in the form of sweet birch oil which you can buy. Another ingredient is safrole free sassafras oil, but I think there are legal difficulties obtaining it in the USA, because a principal constituent, heliotropin, is a controlled substance because it can be used to make the psychotropic drug, ecstasy. Heliotropin is also a constituent of natural vanilla. All good imitation vanilla contains heliotropin, but I have read the labels of many brands of imitation vanilla and all are missing heliotropin. Maybe, the legal red tape for purchasing heliotropin is too much for many manufacturers. I suspect the vanilla like accent of A&W rootbeer might be heliotropin.
My favorite rootbeer of all time was Fanta Rootbeer which is no longer available except maybe at soda fountains. It had a good, solid, distinct flavor. When it was available, the label said that it contained all artificial flavoring. I wish I knew the identities of those flavorings. Anyway, if I remember correctly, Fanta is owned by the Coca Cola Company which also makes Barg's Rootbeer. The Fanta Rootbeer recipe is probably sitting idle in some vault.