I believe that the below are under researched. Hence, intensively reading literature on the listed areas can help someone come up with a wonderful research topic.
1. Servant leadership
2. Transactional leadership
3. Transformational leadership
4. Cultural influence on leadership
5. Comparative analysis of leadership in different settings
Sir, I think you need to find an aspect of leadership that particularly interests you as you will need to maintain that interest over several years, I guess. Leadership literature is vast now, so perhaps find a niche area, for example, 'ethical leadership'. I am sorry I cannot offer any further advice. I also aspire towards completing a PhD but have not yet had the opportunity to do so. I am currently working towards an MBA. I am sure you will find others using ResearchGate who will have direct experience with a PhD and may be able to provide more specific advice based on their personal experiences. I wish you all the very best in your academic pursuits.
Leadership is such vast topic and has so many conceptualizations. In fact, you might want to see how leadership is conceptualized in your own field. How do leaders develop? What are the significant developmental experiences that lead to successful leadership practices? What competencies do subordinates want to see from their leaders? Do these competencies differ from the competencies that leaders want to develop? You can look at these things from different contexts. Would conceptualization of leadership differ between countries, between industries, between companies?