The sky is the limit. I prefer to use imitation crab legs in my sushi bake because I don't eat seafood and it's precooked! Just about any fish or even chicken would work great though! If you're not using pre-cooked protein, and it's not sushi grade, just make sure to cook the sushi casserole in the oven until your protein is cooked through to be safe! That's why most versions of this recipe are called a Sushi Bake!