i. Pick a complicated topic, but focus on simplifying it. If you pick a complicated topic, but focus only on demonstrating its complexity, you’re likely to lose most of the audience, which won’t make you appear smart, but rather arrogant or desperate to appear clever or smart.
On the other hand, if the goal of the talk is take a complicated topic and to demonstrate how a layman can understand it, you are likely to gain widespread interest from the audience. Even among an academic audience of experts, they will appreciate it if you can explain your topic in a way that all people—not just someone from your particular area of expertise—can understand.
ii. Pick a simple, but controversial topic. Another great way to appeal to a large audience is to focus on a controversial topic that is simple. Each person in the audience will have opinions about the topic coming into the event, which will make it hard to ignore your talk—especially if you do a good job of highlighting and discussing important issues.
iii. Propose a new solution to an old problem. A final way in which you can capture the attention of a large audience is to propose a new solution to an old problem. For instance, you might focus your talk around a well-known problem (perhaps the government’s inability to pay for a particular program), and explain how you believe it can be solved in a creative, novel way. This should help you to gain interest from a large audience.
Source of Information : Public Speaking Exposed