UPDATE -use dill unless you don't like dill. And
then add some of the other things you'll see on the internet
AFTER you've made the basic recipe.
Another ingredient that I just remember seeing was a substitution of cheddar cheese for feta cheese.
Please don't do that. An "Authentic Spanakopita" recipe may not exist but cheddar cheese is not a good substitution for spinach pie/Spanakopita.
Try my recipe or KK's recipe (hopefully she'll put it up).