You already have the basis of your request in your posting. The list of theme/topics you wrote is a great place to start this assignment. Whenever you think about a piece of literature, you should be asking yourself, "what point is the author trying to make?" The answer(s) to that question are the statements of theme. The next thing to ask yourself is "how does the author make the theme evident to the reader?" The answers to that question will require you to consider all of the literary techniques an author has at their disposal. Start with the essentials: plot development/conflict and characterization. Then consider other literary devices such as language, tone, symbolism, structure, etc.
For each of the above topics you could ask your group, "What is Oscar Wilde saying about _______?" What is he saying about marriage? How is that revealed through the differing attitudes about marriage as revealed by Jack as opposed to Algernon? What does Lady Bracknell's behavior reveal about what is important in a marriage and how married people should behave?
What is Oscar Wilde saying about manners? Who behaves rudely? How is the behavior rude? What social mores are shown in the play? What happens to the characters that break the rules of society?
What is O. W. saying about gender? What is considered appropriate behavior for the women? How are they treated? How are they characterized? Are they characterized as smarter or more foolish than the men? What are their ambitions? Are they taken seriously by the men? What are their strengths and weaknesses? What are some differences between Lady Bracknell and the younger generation of women, Cecily and Gwendolyn?
What is O.W. saying about artifice? Who acts this way? Why? What do they gain for this behavior? What are the threats to the artifice? How does the artifice showcase a level of creativity or inventiveness? Is the artifice admirable? Necessary? How? How does O.W. use witticism here?
This is such a fun play, and there are so many things to talk about, I can't imagine you will have trouble being a discussion leader.