Thank you all. We now have 12 people, so I have launched the "Quest for Great-Grandparents: Fizzy Edition" thread.
But by all means keep on looking, because I'm thinking that we should be able to do a Fizzy Edition Part Two once we have profiles for all the people on this list. And possibly a Part Three, if we find even more.
I was kind of frustrated in my research, because root beer, cream soda, ginger ale, and ginger beer (or cola, for that matter) seem to have been invented by multiple people in multiple places and times, so we can't point to single inventors for any of them. But, I suppose it's even better to put up profiles for all the different inventors and then try to connect them and add their great-grandparents.
I belatedly thought of, and then couldn't find, a guarana-flavoured pop called Tuba that I encountered in Hong Kong. Apparently, it sank without a trace, but Wikipedia does list two similar products, and their inventors, and that gives us a couple of other people to research:
I may write to the companies concerned, and see if I can get at least names, birth dates, and birth places for the inventors of Fresca and Mirinda.
Finding out anything about Keniada (a particular favourite of mine that I've never found anywhere else) is probably going to be a real challenge for anybody who doesn't live in Bosnia.