I've done needs assessments for disaster case management, but not a healthcare system. Here is a link to an article that may be helpful https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1113037/
Adopt first a definition of need (May be the gap between desired state and current state of the target population. This can be measured using specific indicators of morbidity, mortality, services availability, services utilization...etc) plus some sociodemographic aspects of the target population. Then decide what changes do you want, expect or desire to achieve within a given period of time. These achievable changes require now the resources (health care services) to be established or added. You need the help of an expert epidemiologist, manager and advice from an economist