I think when you're making this dish, you really only need 4 basic ingredients: Potatoes, cheese, onions (unless you don't like cooked onions), and heavy cream. An optional topping like bread crumbs or panko could be added as well.
With that, I think the secret is cutting the potatoes and onions as thinly as possible (if you have one, use a mandolin) and use a variety of good cheeses. My favorite au gratins are always made with fontina, gruyere, cheddar, and maybe another cheese. I think adding the blue cheese would be amazing! Layer that baby up and bake!