Basic carrier sauce/syrup for different sweet flavors?
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.