Sake is added to the marinade for 4 reasons. The first is that sake contains a ton of amino acids, which creates the taste of umami. The second is that sake, which is made with rice, imparts the rice's unique flavor to the chicken. The third is that it contains some residual sugars which lend a mild sweetness to the marinade without making it sweet. Finally, the liquid will dilute the soy sauce, preventing it from getting too salty.  Unfortunately, there is no great substitute for sake that hits all four of these factors and without introducing unwanted tastes to the dish. For example, the chicken stock will add amino acids and help dilute the marinade, but it won't add rice flavor or sweetness. Fruit juices are too sweet, and the acidity will make the chicken sour. Wine can have the amino acids, and residual sugars, but like fruit juice, it's acidic, which is not desirable.  Although it's not going to taste the same as using sake; chicken stock or dashi with a small pinch of sugar is the best substitute.