As Adrian stated above, past projects are a source of design inputs. To expand on this, the Design History File (DHF) of other similar projects would be a source of design inputs. The key to choosing design inputs is to find something that satisfies the customer but is also practical. The customer doesn’t have the history of designs that failed or why they failed. Its your job to look at similar projects and if something that the customer is suggesting failed before, don’t use it. You have to take the customer’s wants into account but as an engineer, it is imperative that you put practicality into account first. When I say practicality, I mean pure science, safety, and most importantly things that you might not have thought would be an issue or lessons learned from past projects. This information would be in the DHF of these past projects and would be a great source of design inputs.