Basic carrier sauce/syrup for different sweet flavors? I have an application where I would like to have a half dozen different flavors (all sweet) that are in the form of a sauce or slightly viscous liquid.

Rather then 6 recipes I am hoping to find a sauce base that I can flavor with different flavors.
i.e. Peppermint, vanilla, ginger, cinnamon, dark chocolate, etc..

Mouth feel is important.

I was considering a stock syrup but am concerned that it might be to sweet or not have a good mouth feel and would like to thicken it somehow
Creme anglaise is probably ideal for this purpose in many desserts.  It is basically a very thin custard.

You can melt or infuse your flavor ingredients into the cream before using it to make the sauce.

Chocolate is a special case--you may want to use specific chocolate sauce recipe, as it will effect the texture of the sauce, and change its properties.  Still, chocolate creme anglaise is possible, as is a simple chocolate ganache.