The problem with other ingredients is that they may be “Mevatel” the essential ingredients, or they may create a Chatzitza (interposition) between the mouth and the sufganiyos, or they may it hard to eat a Kezais in the required time.
We need a comprehensive Artscroll sefer that will deal with all these details, with good Mar-eh Mekomos (sources listed) and several Haskamos (approval letters) from recognized authorities.
Hamentashen are next.