Can you please give me some ideas on what to write about in a business letter suggesting something not complaining?

It depends on where you want to write to. You could suggest to coffee shops that they hire someone to take phone orders of people within 5 minutes of the store who want to pre-order their drinks and pick them up at a special rapid drive-thru. You could suggest a new item to be added to any food business's menu. You could suggest to technology-focused businesses a feature that would make using their computer or phone programs easier. Or if you like a certain style of clothes but can never find it, write to a similar designer and maybe they will realize there are more people in the world with your problem. They have people who are paid to think of new ideas and product development, but your ideas may be different than what they can come up with. Basically, you could suggest changes in options offered, methods of communcation with the customers, or how the business works. You could also criticize them, but constructively, and use a fault for a basis of a suggestion that you come up with. The fault could be a great source of inspiration. Just think of a product you use a lot, or a store you go to, and think about what you would change if you could.