Spent the morning looking for discussions that appear universally popular.
Four categories stand out; doing, thinking, using, and reading (or watching).
Members want to know what people like themselves are doing, thinking, using, and reading.
First, let's begin with doing. Members want to know what people like themselves are working on, struggling with, what they plan to do in the future, what jobs/occupations they have, what they do in their spare time. Discussions around these topics are universally popular.
Second, thinking. Members want to know what other members thinking about the most popular/controversial topics. Sometimes it's a genuine interest, sometimes they're just looking for a lively debate. They also want the knowledge that other members have. They ask questions to unlock that knowledge.
Third, using. Discussions about tools and equipment are forever popular. In almost any topic, we need to use something to help us be involved. Discussions to identify the best tools or comparisons between popular tools, or about specific situations are universally popular.
Finally, reading. We want to read what others are reading. We want book recommendations. Sometimes we want film and music recommendations. Mainly, however, we want the book recommendations.
If you're looking for discussions to highlight or initiate, any of the above should get a good response.