I never used any tool for topic researches for Webinars and Podcasts. Do resource research such as Ebooks, Webinar topics, Infuencers focus in your nieche, etc in your domain.
For Podcast make it interactive.
1. How did you get your first 100 customers
2. How did you get to your first 100,000 USD revenue
3. How did you bag your first big client
4. How did you build your sales team/tackle sales
5. How to manage remote clients