An American stout is a relatively good base style to add additions to, with quite a range of possibilities. The hop forward character of the American stout makes it somewhat more challenging, and less of a blank canvas, but there are still some very good options.
Spices consistently rank high, including seeds of paradise, chai and cinnamon. The roasty character pairs well with other roasted ingredients, like coffee, and to a lesser extent, chicory and chocolate.
Bourbon and whiskey go well with the American stout, although a stronger stout may support the booziness better.
Fruits are a mixed bag- raspberry is an unexpected leader, with orange peel and blackberry coming in a bit lower.
There are a few controversial combos that have potential, but are likely to be strongly disliked by some. These include mint, peanut butter, elderflower, or juniper berries.
Tart flavors like rhubarb, hibiscus or cranberries are not a good match, as the roastiness does not seem to go well with sour.
BJCP Style: 20B