Butter cream icing would be simple. Real life recipe is just 1lb or 4c powdered sugar, 1/2c or 1 stick butter, 1/2 c shortning, 3tbsp water, & 1tsp vanilla extract. I make it at least once a month. Just made it last week, lol.
So in flavorings alone, I think it would be butter, a hint of vanilla, & sweetener, though I'm not sure how the ratios would work because you wouldn't want the sweetener to overpower the butter & vanilla.