I would think that caffeine would be a viable ingredient. There are studies that show caffeine doesn't actually have any flavor to it, so could potentially be added to anything. (Its just a case of if you want something with caffeine or not.) http://www.nlm.nih.gov/medlineplus/e...cle/002445.htm
This means you can potentially caffinate any soda you would ever want. Even plain caffinated water (fizzy or non).
Generally speaking, there's two ways to go about soda. There's the co2 tank process, usually using either a keg, or a carbonator cap
and 2 liter bottle, or there's sugar and yeast.
That will carbonate the liquid. After (or before/during) is just the flavorings, wether you want to create your own by some sort of method, or to use a flavor extract/syrup.
Carbonating by co2 tank is pretty easy, just carbonate the keg or 2 liter and use it either as is, premixed (before carbonating) or after by using carbonated water and flavor syrup. People tend to use upwards of 12 psi, (usually around 35-45 psi depending on the desired carbonation).
Using yeast and sugar tends to be harder. Even harder than beer. With beer, it's generally that the yeast will consume the priming sugar and produce co2 and perhaps a bit of alcohol, then stop because the sugar is consumed.
With soda however, because there tends to be a lot more sugar, you have to be aware of the current pressure (usually by taste test, squeeze test on a plastic bottle, or a pressure gauge) to find out when you think it's ready and cold store the rest and drink it pretty quickly (since it would still carbonate further in cold storage, just slower than if it were 60 degrees).
Alternatively, you can carbonate it as if it were beer when using non -fermentable sugar, so that when yeast finish the sugar, it stops eating anything else. (again, this is similar to priming sugar where x amount of sugar will generate Y amount of carbonation. Refer to your beer charts for this) This method is useful when sweetening a soda without sugar (using splenda/sucralose, nutrasweet/aspartamine, zevia/stevia and sweetnlow/Saccharin. Maybe xyitol?