Well trending topics are called that because they are Trends and Trends do change. A good place to look for trending topics on YouTube to talk about would be the
YouTube Trending page. That page features YouTube Trends by current Interest. Another good place to see what's trending and what people are searching for currently is
Google Trends. Google Trends shows what people are searching for the most at a given time through Google. And the last way I can recommend is to just watch YouTube videos. If you notice a trend you can think of jumping on that bandwagon too!