If you're not into seitan, you've got a few options. First, you could simply omit it, though you will end up with less filling and fewer pot pies (so cut back on the pastry as well). You could substitute mushrooms (crimini would be nice), tempeh, or veggies (try root veggies like carrots).
*I used 5.5 inch, 16 oz. ramekins, which yielded 4 pot pies. Use smaller ramekins to get 5 or 6 pot pies.