Rajaa A. Mahmoud +1 if you choose a national approach.
I would suggest you a Health public approach with a contextual problem/action in your country.
Example 1: community health means actions with health professionals, ONG, patient association,... lot of people. How the best practice for manage a trans-organisation program ?
Example 2: You can focus your study in a hospital context... how manager in hospital respond to a contextual problems like turn over, burn out, understaffed service, insufficient material, epidemic, supply chain operation ?
Example 3: The interface between hospital and free clinic... Which factors can improve the management or which different management in both situation ?
Example 4: Evaluation of health public programs base of different situations (geographic, economic, diversity...)
Example 5: The use of new technologies in Health management disease.
Example 6: Compare manager in health and other domains. Which formation ? age ? gender representation ? moral value ? specific challenge ? skills ?...
I suggest you look at public health research from specific areas which would lead to a more focused outcome. For instance, there are gaps in research on young people's mental health, you could take an organisational approach to the implementation of mental health services for young people.
As a project management consultant working in the field of healthcare, may I suggest you might want to look at community health development projects. There are a lot of these projects either government led or NGO driven world-wide where projects from implementing mobile health devices in rural areas to digital health programmes in urban areas or community health education projects empowering women to manage HIV medication in the home in remote areas. It really depends on what research angle you wish to take.
It may be helpful to look at some of the quality initiatives on the WHO web site as these could give a standard to use. For example, Baby Friendly Hospital Initiative has implementation guidance plus many publications on how countries/individual health facilities used project management / quality improvement to implement the Initiative over the last 25 years. A very wide range of topics on the WHO site - medicines, communicable diseases, older persons, air pollution (very current topic) and any more.