There are plenty sources of input to pool from when developing the Design Input Document (DID). Other than the marketing department you should be collecting information from external such as consumers and doctors. Consumers can be useful by supplying knowledge in the form of surveys and focus groups to see what types of features in a device they would be interested in. For doctors, sales representatives should be spending their time in hospitals communicating with the doctors to establish performance deficiencies and which tools need improvement. They could also be sitting in on procedures and analyzing what they feel the doctor is having a tough time with. By gaining information from patients and doctors you can cover the entire spectrum of the product and determine where to allocate resources to improve the design.