I'm thinking along the lines of a Belgian monestary raspberry ale with something along the lines of a local gellato's chocolate offering. That is based only on the theoretical taste combinations.
In real life I fear that the ice cream will curdle in the beer, or destroy the carbination and head necessary for good beer taste.
Now, if we are talking root beer (not the crap most places sell, but a real home-made old-fashioned root beer) the only thing to put in it is chocolate ice cream. A&W ruined most folks' taste for a root beer float by limiting the choice to vanilla ice cream or nothing.
stay safe.
skidmark