There is no point in discouraging u from seeking answers. To decide about this we may have to ask number of questions,
What is the method of your business. are you selling to public or only to corporates.
if public which are the places. if corporates are you entangled in the agreement. What are the main menu - only snacks or lunch, dinner or both.
However I suggest the following methods to decide.
1. Find out from the existing customers what about their opinion on the existing menu. which are the items they like most and which are not according to their taste.
2. What are the possible improvement to be made in the existing menu in the angle of the customers. get it from them.
3. What are the new dished they expect from you. specify the items.
4. The categories of customer pay a visit to your fast food.decide upon based on this information.
5. Give importance to the side dishes which are mostly liked by the customer, Improve on it.
6. Give a sample of the new dish to the existing customer free of cost ( small qty) and get the feed back and introduce the most favored dishes initially.
7. Compare with other fast foods and get the list of items which are supplied by them as favorite dishes and not by you. make a research onthat and try to introduce such items slowly one by one.