I made this simple WASC variation for cupcakes, but they had a lovely texture and would be perfect for layer cake as well. The pumpkin in the recipe is basically replacing the sour cream. It ends up having a nice mild flavor, but you could add more pumpkin if you wanted it stronger.
1 box yellow cake mix
1 cup all purpose flour
1 cup granulated sugar
1 1/2 teaspoons pumpkin pie spice
3/4 teaspoon salt
1 cup pumpkin (not pie filling)
1 1/3 cups water
2 tablespoons oil
2 teaspoons vanilla extract
Combine all ingredients on low until incorporated to avoid splashing, then increase speed to medium. I generally mix for about one minute, but do whatever you prefer.
Bake at 350 degrees until cake just barely springs back to your touch, or until a toothpick inserted in the center comes out clean.