How can I get a syrupy "mouth-feel" in sugar-free syrup? I viscerally hate low calorie sugar replacements - all of them, including Splenda (sucralose), except in one application. In my iced coffee I like sugar-free hazelnut syrup. The brand that I've been using is sweetened with Splenda (and sneakily, acesulfame potassium). It would be great except that it's very expensive. I spend $70/month just for that syrup. I have 50 grams of sucralose (that's enough to sweeten my coffee for years) which is the sweetener in Splenda. I also have hazelnut extract. I've made a "syrup" with water, sucralose and hazelnut extract and it turned out OK, but the coffee drink really lacks something without the syrupy quality of the commercial stuff.

The ingredients on the label for the commercial syrup: Purified water, natural and artificial flavors, citric acid, acesulfame potassium, sodium benzoate and potassium sorbate (to preserve freshness), xanthan gum, sucralose (SPLENDA Brand), caramel color.

I know that acesulfame potassium is a sweetener, Splenda generally contains maltodextrin and sucralose, but that doesn't seem to be the case here. There doesn't seem to be any maltodextrin in the product. Unless I'm missing something, that means that the commercial product gets its viscosity from an infinitesimal amount of xanthan gum. Could that be right?

Of course I can get xanthan gum. Say I'm making 2 cups of syrup at a time, I add 1/8 tsp sucralose and two teaspoons of extract to 2 cups of boiling water (off heat). Now what?
I don't have a good suggestion for what to use, but at least I can give you some info on xanthan. 

Yes, syrup can take viscosity from an infinitesmal amount of xanthan gum. If you add 0.5% to 1% of the fluid's weight in xanthan, you get a pudding consistency. For a syrup-like viscosity, you need much less. 

But xanthan is not a sugar, and does not make a syrupy consistency. It makes stuff gooey, not sticky. This may be enough for you, if all you need is some thickness, but the texture won't be the same as normal sugar syrup. I haven't tried commercial sugar-free syrup, so I can't make a comparison there. 

Xanthan has also the unfortunate tendency to reduce aroma, although it may not be a problem in the little amounts needed for syrup-like thickness.