I had heard of adding vodka to the batter for dipping zucchini blossoms before you fry them, and it does seem to boost the crispiness.
But I agree with mar25. Cornstarch is the only thing I’ve ever considered adding to my flour/salt/pepper dredge, and I didn’t find it made that much difference when I did. So I stopped doing that, too.
I presoak the chicken in a brine of water, salt and baking soda. I don’t know why the baking soda; I read it in the cookbook that accompanied the old PBS series, “Great Chefs of the West,” and have used it ever since.