Emulating Starbucks Frappuccino Cream Base
I can't comment yet...but if I could, I might suggest using or swapping a portion of your sugar for liquid invert sugar, instead of caster or white granulated, in conjunction with the afore mentioned artificial flavorings, and possibly some more milk protien like sodium caseinate.  Invert sugar is often used in candies that must maintain a soft creamy liquid center (like those queen anne cherry bon-bon things), and I think it would positively alter the end-texture.  I bought some in Austin at a local cupcake/candy bakeshop that wasn't very expensive.(www.allinonebakeshop.com call them,  the website doesn't reflect all of the cool products they have). 

Hansson et al experimented with different sugar compositions for their beverages and noticed that there was a greater release of specific aromatic compounds using invert sugar. They posited that the glucose-fructose ratio that liquid invert sugar possesses, causes the water molecules to be more structred, thereby decreasing the amount of free water around.  Hyrophillic(water loving) flavors in the mix are then more concentrated in the available free water, waiting to be tasted. The hyrophobic( water-hating/fat loving) flavors like to stick with the fats, so using a very good emulsifuer to effectively distribute/suspend the fats through the rest of the mix will be important for the end flavor profile.
doi:10.1016/S0308-8146(00)00243-0

I might even add some turmeric to your mixture when you heat it (be aware of some yellow color transferrence), since the curcuminoids (bioactive ingredients in the turmeric rhizome/powdered spice) decompose into a few compounds like vanillin and ferulic acid when heated (longer the incubation = more vanillin as a result of its degradation). Here's one research paper doi:10.1016/S0731-7085(96)02024-9 I'm happy to provide additional peer-reviewed sources if desired.

One last thought on flavor additions...if you dont mind artificial, this is a gluten-free butter flavor:  http://www.shanks.com/product-category/extracts-flavors/imitation-butter/  It almost goes without saying, that only a VERY small amount would be needed(maybe a drop or two at most to really round out that creme profile).