What do you already know? What do you already do well?
What do you want to do? What are your intrinsic interests? How do you actually spend your disposable time?
Exercise: ask others "what do I do well?" How well do their perceptions match with your own?
What “personal strategic advantage”(s) do you have?
Use mind / concept mapping to display a graphical view of your interests. what are your core interests? how do your interests connect? what interests overlap?
Know your world:
What do your employers, friends, colleagues, and potential clients want or need to know? Why?
Could you help solve an important problem or otherwise significantly improve or protect their businesses or their lives?
What’s your connection with them? Will they trust you?
Ultimately, you're looking for for that sweet spot where your interests and experiences overlap with areas of demand for expertise. You'll want to leverage your existing knowledge, skills, connections and experience to maximize both intrinsic interest in a subject and extrinsic demand.