Start and end always with something you are passionate about. It doesn't have to be your field of interest.
When I was working as an international consultant, the policy of the firm was to place you in a field other than your field of expertise. Their reasoning was to expand your breath of knowledge across different industries.
I have taken their lead and have ventured forth beyond my main field. I am passionate about history and have begun restoring a vintage home so I just started researching something I know nothing about - vintage porcelain and ceramic tiles.
The beauty of HubPages is you don't need a niche. If you have your own website, your own blog, you must position yourself to gain traffic. Here on HubPages, the team promotes all subjects. Cross promoting in the same subject area with the links is what Google wants.
One simple rule - write what touches your heart and speaks to your soul.