A thesis is the departure point for your entire discussion, so it’s important to pick and shape one that sets up workable parameters for you.
The first key element to understand is the difference between a single-thesis and a multi-thesis discussion (see: Multi- vs Single-Thesis Discussions). This is not just a structural difference: different discussion questions are suited to different types of discussion.
In general, multi-thesis discussions are better suited to open-ended questions (e.g. ‘Where should I go on holiday?’, ‘How should countries handle the legality of mind-altering drugs?’ or ‘What changes would improve employee morale?’). Single-thesis discussions are better suited to direct proposals that prompt yes-or-no answers (e.g. ‘Is New York a nice holiday destination?’, ‘Marijuana should be legal for recreational use’, or ‘We should have a staff movie night’).
You should also think about the scope of the discussion that you want to have. Big, sweeping questions are going to lead to larger and more complex discussions, at least in general. If you want a quick, fun debate with friends, you may well want a more constrained topic, to give you a realistic chance of canvassing all issues. By contrast, a debate that’s too narrow and specific may not give you anything to say beyond a few shallow points!
Topics and theses need to be interesting to your intended audience - that usually requires them to be reasonably challenging and topical questions, and largely rules out personal dilemmas (unless your intended participants are friends and family).
Beyond just coming up with a good idea for a topic or thesis, it’s important that you also express it in a way that effectively communicates your intended discussion to other participants. Think about how a reader will interpret the wording of your thesis - is their ambiguity about what the discussion is intended to be about? Generally, it is better to keep the language of theses and topics direct and to the point - you can always make use of the background information box (see: Adding Background Info) to define key terms.