When choosing a niche, you should focus on the following THREE characteristics:
1. What you like to work on (not necessary if you are hiring a writer)
2. Where you can compete and win (the market needs to be penetrable)
3. What has enough audience to buy your offers (freebie seekers don't make you money)
I would focus more on #2 and #3.
Passion is overrated. Once you become good at a particular subject—and get paid for the same—you will start to love it sooner or later. It's just like falling in love with a person. Even if you don't like that person, the more time you spend alongside the person, the more you interact with the person, positively or negatively, the higher the chance of you developing strong emotions for him or her.
Now, doing market research can be gruesome, my friend.
I would suggest following the philosophy "inch wide, mile deep" to start with.