As of September 2014, YouTube's own keyword tool stopped working. It was moved over to Display Planner's AdWords video keyword suggestions. To use it, you need to have an Google Ads account. Just like the non-existent YouTube keyword tool, it was created to facilitate paid video and ads promotion. Which is why keywords that you will find there might not be closely related to the topic that you want to cover.
The tool also helps you discover and follow the hottest topics in your niche via the nifty hashtag reports. You can see which hashtags are making posts more engaging and take a peek at what your competitors are doing. Plus, it allows you to see who has mentioned you, your brand or products with a comprehensive “Mentions” report. The report also helps you discover PR opportunities.