Think like a kid. The toy industry has a voracious appetite for new ideas. I should know because I work with all of them. The problem is, the toy companies usually don't know what they are looking for. You can always try to think of improvements on a company's existing products. They are always looking to 'refresh' brands.
To invent a new toy think about play patterns. Play patterns usually consist of: Active play, Pretend Play, Cooperative play, Competitive play, Heroic play and Transformation play. If you can come up with a new toy that fits any of these play patterns, you might have a winner.