To some degree, a person that does have access to a soda fountain can create his or her own sodas from a combination of what syrups are available at that location.

For actual self design and mixing of soda concoctions in an as-needed retail enviroment, it is technologically doable, but marketwise, the majority of the public wants premixed familiar branded flavors, of which due to recipe cofidentiality, must come at mimimum as a ready to mix (with water) syrup.

If you added a "health" benefit to this idea you might find a wider market. Folks could add various vitamins or other supplements to make their standard coke a "boost" of sorts.

Maybe instead of a soda fountain, this would be a Mixed Drink fountain for use at bars and public events where the turnaround is neccessarily high. A series of positions for glasses is provided (station A, B, C, D, & E). The pseudo-Bar Tender selects the drink desired by the customer, reads the requirement for the glass size and condition (iced, salted rim, etc.), then sets the glass in the station and presses a button over that glass. The appropriate combinations of ingredients are proportioned properly and put in the glass. While this is going on the pseudo-Bar Tender is preparing the next glass. This would probably allow one person to do three times as much business and the drinks would be perfectly proportioned every time.