Hot Burger-long Burger suitable for putting in hot dog rolls.
Pitta Burger-shop bought or home-made Pitta bread’s possibly seasoned.
Traditional Quarter Pound Burgers-differentiated with different and better quality burger buns (similar to the option of different breads offered by Subway).
Augments to the Burger:
Rather than try to change the flavour of the Burger pre-cooking which would lead to additional stockholding needs options include:
Spicy powder coatings which can be cooked into the Burger.
Using condiments to add variety and flavour (again similar to the Subway use of different sauces).