Vanilla ice cream in Jagermeister
Orange sherbet in Bailey's Irish Cream (it's like an adult dreamsicle).
As @Rory mentions, ice cream cocktails automatically become sweet and creamy, which is why I find sticking with liquors that you expect to be sweet and creamy pair very well.
Per your original mention of the classic root beer float, I ...
One of the great ones we grew up with was the Cider Spider - a cider float with vanilla ice cream, using proper cider, not the strange thing that seems to pass for cider in the US. I assume it came from Australia, as it was my mother who introduced us to it.
But since then experimentation has showed that most basic ice cream flavours can work well in fairly ...