I've got a couple of sweetener-free recipes I use with my soda maker, though I should warn you nobody else I've given them to likes them. But I like them well enough.
One is a couple of squirts of vanilla, a couple of squirts of lemon juice, and a bit of salt. Salt is probably an underappreciated drink ingredient for this sort of thing. It turns out it isn't in your soft drinks just to make you want to drink more. This makes something that is related to cream soda, except for the aspects of cream soda that come from being crammed full of sugar, which I can't do much about.
I also have a mix I keep around made out of 3 tablespoons salt, 1 cup vanilla, 1/2 cup lemon juice, 1/2 cup lime juice, and about 1/3rd cup almond extract. I measure it all (except the salt which I just put in directly) into a single 2 cup Pyrex dish and just sort of eyeball the last 1/3rd cup of almond extract, then funnel it in to a holder. I use McCormick 32 oz vanilla and almond extract for this and order bulk RealLemon and RealLime juice for this from Amazon, and mix it into one of the leftover bottles and keep it around refrigerated. 3 squirts and "whatever dribbles in" as I'm removing the bottle is what I used for one DrinkMate bottle. To taste, as all of this is, of course. If nothing else this is pretty cheap per drink.
You can also mix unsweetened electrolytes in, but you have to wait until after you dilute the mixture with water or it'll react with the lemon & lime juice. Salt you can keep in the mix but not electrolytes in general. It adds a certain body to the mix even if you're not interested in the electrolytes per se, and a single packet of them lasts a long time.
You're not going to go into business selling this stuff, but if you're already drinking unsweetened apple cider vinegar & lemon/lime juice as a beverage flavoring we might just have some compatible tastes here. Carbonation is required, though, otherwise the vanilla and the almond extract don't come through at all.