You can't go wrong with evergreen markets like health, wealth, and relationships. People will ALWAYS have these problems.
Just choose one category and narrow it down from there.. like health, there's weight loss, getting in shape, getting rid of acne, getting rid of wrinkles, hair loss, back pain. Seriously just choose any one of these and it'll be profitable.
Or you could keep narrowing it down if you want to create a product such as - weight loss for women who just had a baby.
If you need more ideas, go to 43things and check what a lot of people are trying to achieve:
Code:
http://www.43things.com/zeitgeist/popular_goals