There is a rather popular "Campfire Stout" made by Highwater Brewing.
I'd suggest a medium bodied, 6% to 7% abv, oatmeal stout base. After fermentation, add some cocoa nibs and split vanilla beans, then rest until their flavor becomes prominent. Before bottling, add lactose to your desired sweetness level. It should suggest s'mores, rather than taste like a dessert.