You could add a few blueberries or raspberries to the vegan chocolate mug cake batter. This will likely make the cake wetter, with more of a 'lava' cake effect. You could also spoon in some nut butter to the center of the batter - such as peanut butter, almond butter, or even Vegan Nutella.  For a chocolate caramel swirl then simply spoon in some vegan caramel, or use the nut-butter based caramel from this recipe.  Add a few drops of natural flavoring such as peppermint oil or orange oil to boost flavor further for your vegan mug cake.  Add a spoonful of crushed walnuts or other nuts, if preferred, or even some shredded coconut.