Officially your not suppose to add anything but I have cheyenne, black, ground and white pepper also paprika, smoked paprika and hot paprika (from Hungary) , chilli powder and finally Garam Masala. But never add salt, also be very very careful with some spices as they contain citrus bits and pieces.
ive got a cupboard full of herbs and spices and if the same rule applies as tea, then theres a lot i can use, so no salt, pure spices and herbs only, and no citrus, got it!